Process and apparatus for manufacturing aromatic carboxylic acids

ABSTRACT

An improved process for preparing aromatic carboxylic acids by the exothermic liquid-phase oxidation reaction of an aromatic feedstock compound wherein water is efficiently recovered from the exothermic liquid-phase oxidation reaction and treated to reduce corrosive agents residing therein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSerial No. 61/917,465 filed Dec. 18, 2013, and entitled “ImprovedProcess and Apparatus for Manufacturing Aromatic Carboxylic Acids” whichis hereby incorporated herein by reference in its entirety for allpurposes.

FIELD OF THE INVENTION

This invention relates to an improved process for preparing aromaticcarboxylic acids by the exothermic liquid-phase oxidation reaction of anaromatic feedstock compound wherein water is efficiently recovered fromthe exothermic liquid-phase oxidation reaction and treated to reducecorrosive agents residing therein.

BACKGROUND OF THE INVENTION

Terephthalic acid and other aromatic carboxylic acids are widely used inmanufacture of polyesters, commonly by reaction with ethylene glycol,higher alkylene glycols or combinations thereof, for conversion tofiber, film, containers, bottles and other packaging materials, andmolded articles.

In commercial practice, aromatic carboxylic acids are commonly made byliquid phase oxidation in an aqueous acetic acid solvent ofmethyl-substituted benzene and naphthalene feedstocks, in which thepositions of the methyl substituents correspond to the positions ofcarboxyl groups in the desired aromatic carboxylic acid product, withair or another source of oxygen, which is normally gaseous, in thepresence of a bromine-promoted catalyst comprising cobalt and manganese.The oxidation is exothermic and yields aromatic carboxylic acid togetherwith by-products, including partial or intermediate oxidation productsof the aromatic feedstock, and acetic acid reaction products, such asmethanol, methyl acetate, and methyl bromide. Water is also generated asa by-product. Aromatic carboxylic acid, typically accompanied byoxidation by-products of the feedstock are commonly formed dissolved oras suspended solids in the liquid phase reaction mixture and arecommonly recovered by crystallization and solid-liquid separationtechniques. The exothermic oxidation reaction is commonly conducted in asuitable reaction vessel at elevated temperature and pressure. A liquidphase reaction mixture is maintained in the vessel and a vapor phaseformed as a result of the exothermic oxidation is evaporated from theliquid phase and removed from the reactor to control reactiontemperature. The vapor phase comprises water vapor, vaporized aceticacid -reaction solvent and small amounts of by-products of theoxidation, including both solvent and feedstock by-products. It usuallyalso contains oxygen gas not consumed in oxidation, gaseous methylbromide, minor amounts of unreacted feedstock, carbon oxides and, whenthe oxygen source for the process is air or another oxygen-containinggaseous mixture, nitrogen, carbon oxides and other inert gaseouscomponents of the source gas.

Pure forms of aromatic carboxylic acids are often favored formanufacture of polyesters for important applications, such as fibers andbottles, because impurities, such as by-products generated from aromaticfeedstocks in such oxidation processes and, more generally, variouscarbonyl-substituted aromatic species are known to cause or correlatewith color formation in polyesters made from the acids and, in turn,off-color in polyester converted products. Aromatic carboxylic acidswith reduced levels of impurities can be made by further oxidizing crudeproducts from liquid phase oxidation as described above at one or more,progressively lower temperatures and oxygen levels, and duringcrystallization to recover products of the oxidation, for conversion offeedstock partial oxidation products to the desired acid product, asknown from U.S. Pat. Nos. 4,877,900, 4,772,748 and 4,286,101,Preferredpure forms of terephthalic acid and other aromatic carboxylic acids withlower impurities contents, such as purified terephthalic acid or “PTA”,are made by catalytically hydrogenating less pure forms of the acids,such as crude product comprising aromatic carboxylic acid andby-products generated by liquid phase oxidation of aromatic feedstock orso-called medium purity products, in solution at elevated temperatureand pressure using a noble metal catalyst. In commercial practice,liquid phase oxidation of alkyl aromatic feed materials to crudearomatic carboxylic acid and purification of the crude product are oftenconducted in continuous integrated processes in which crude product fromliquid phase oxidation is used as starting material for purification.

The high temperature and pressure vapor phase generated by liquid phaseoxidation in such processes is a potentially valuable source ofrecoverable acetic acid reaction solvent, unreacted feed material andreaction by-products, as well as energy. Its substantial water content,high temperature and pressure and corrosive nature due to componentssuch as gaseous methyl bromide, acetic acid solvent and water, however,pose technical and economic challenges to separating or recoveringcomponents for recycle and recovering its energy content. Further,impurities that remain unseparated in recovered process streams canprevent re-use of streams if impurities adversely affect other processaspects, equipment or product quality. As described in U.S. Pat. No.5,200,557, for example, monocarboxylic acids adversely affecthydrogenation. catalysts used in purification processes, with even lowlevels of acetic acid residues such as present in crude aromaticcarboxylic acid products recovered from oxidation reaction liquids beingconsidered detrimental

British Patent Specification 1,373,230, U.S. Pat Nos, 5,304,676;5,723,656; 6,143,925; 6,504,051, European Patent Specification 0 498 591B1 and International Application WO 97/27168 describe processes formanufacture of aromatic carboxylic acids by liquid phase oxidation ofaromatic feed materials in which a high pressure off-gas is removed fromoxidation and treated for recovery and recycle of portions or componentsthereof and, in some cases, recovery of energy. Condensation of off-gas,as in U.S. Pat. No. 5,304,676, is effective for recovery of water,acetic acid and other condensable components of the off-gas butseparating water, acetic acid and other components in. the resultingcondensate is technically complex and economically impractical, Highpressure off-gas separations, as in processes of U.S. Pat. Nos.5,723,656, 6,143,925, 6,504,051 and WO 97/27168, can be effective forseparating off-gases to recover acetic acid-rich liquids and gasescomprising water vapor suitable for further processing. However, certainby-products of the oxidation tend to apportion into both liquid and gasphases in such separations, complicating their recovery and potentiallyadversely impacting other process streams and steps. These difficultiesare compounded by build-up of such by-products in processes in whichby-product-containing streams, such as mother liquor remaining afterrecovery of pure forms of aromatic carboxylic acid from a purificationliquid reaction mixture or liquids condensed effluent gases from highpressure separations are used in separations. None of the processesaccording to the cited patents uses liquid condensed from a highpressure off-gas from a liquid phase oxidation as solvent or otherliquid comprising water in the purification of impure aromaticcarboxylic acids and recoveries of materials and energy in suchprocesses often are accomplished at the expense of each other, forexample due to loss of energy content on cooling or depressurizing torecover materials, burning of materials to control atmospheric emissionsand other losses of oxidation solvent, feedstock and by-products thatresult if a high temperature and pressure vapor phase from oxidation isnot cooled or depressurized for removal of such materials.

Impurities remaining in recycle streams can upset process operation,corrode equipment, and impair product quality. Added equipment andprocess steps for recovering materials, energy or both can add furtherprocess complexities and limit or preclude their practical utility ifthey add costs that outweigh materials and energy savings. Impact ofsuch factors, lost energy and lost materials are magnified by scale ofprocess operations. In world-scale commercial manufacturing plants withannual capacities of 500,000 to 1,000,000 or more tons of product, evenfractional percentages or hundreds of parts per million of feedstock andsolvent lost or converted to undesired or unusable by-products, minorinefficiencies in energy recovery and incremental additions to effluentwater treatment translate to significant practical losses of materials,increases in consumption of fuel or electricity and added processing, aswell as unpredictable process efficiencies and economics due todifferences and variations in costs for energy, materials andrequirements for treatment of gaseous and liquid emissions andeffluents.

SUMMARY OF THE INVENTION

The present invention provide a continuous process for preparingaromatic carboxylic acids by the exothermic liquid-phase oxidationreaction of an aromatic feedstock compound wherein water is efficientlyrecovered from the exothermic liquid-phase oxidation reaction, whichprocess comprises:

(a) oxidizing an aromatic feedstock compound to an aromatic carboxylicacid in a liquid-phase reaction mixture comprising water, alow-molecular weight monocarboxylic acid solvent, a heavy metaloxidation catalyst and a source of molecular oxygen, under reactionconditions which produce a gaseous high pressure overhead streamcomprising water, gaseous by-products, and gaseous low-molecular weightmonocarboxylic acid solvent;

(b) separating in a high efficiency separation apparatus a solventmonocarboxylic acid-rich first liquid phase and a water-rich secondliquid phase comprising dissolved oxygen and methyl bromide; and

(c) reducing the amount of at least one of dissolved oxygen and methylbromide present in the second liquid phase providing a treated secondliquid phase.

In one embodiment the step of treating the second liquid phase comprisesflashing the second liquid phase.

In one embodiment flashing the second liquid phase results in a drop inpressure from about 5 kg/cm² to about 40 kg/cm² to about atmospheric orambient pressure.

In one embodiment flashing the second liquid phase results in a drop inpressure from about 10 kg/cm² to about 20 kg/cm² to about atmospheric orambient pressure.

In one embodiment the amount of dissolved oxygen in the second liquidphase is reduced from an amount of about 2.2 ppmw to an amount of lessthan about 1.0 ppmw after flashing.

In one embodiment the amount of dissolved oxygen in the second liquidphase is reduced from an amount of about 2.2 ppmw to an amount of lessthan about 0.5 ppmw after flashing.

In one embodiment the amount of dissolved oxygen in the second liquidphase is reduced from an amount of about 2.2 ppmw to an amount of lessthan about 0.05 ppmw after flashing.

In one embodiment the amount of dissolved oxygen in the second liquidphase is reduced from an amount of about 2.2 ppmw to an amount of lessthan about 0.006 ppmw after flashing.

In one embodiment the amount of dissolved methyl bromide in the secondliquid phase is reduced from an amount of about 0.03 ppmw to an amountof less than about 0.02 ppmw after flashing.

In one embodiment the amount of dissolved methyl bromide in the secondliquid phase is reduced from an amount of about 0.03 ppmw to an amountof less than about 0.01 ppmw after flashing.

In one embodiment the amount of dissolved methyl bromide in the secondliquid phase is reduced from an amount of about 0.03 ppmw to an amountof less than about 0.009 ppmw after flashing.

In one embodiment the amount of dissolved methyl bromide in the secondliquid phase is reduced from an amount of about 0.03 ppmw to an amountof less than about 0.006 ppmw after flashing.

In one embodiment the step of treating the second liquid phase furthercomprises a step of steam stripping the second liquid phase afterflashing.

In one embodiment the step of treating the second liquid phase furthercomprises a step of stripping the second liquid phase with nitrogenafter flashing.

In one embodiment, the treated second liquid phase is suitable for useas liquid comprising water in one or more steps of a process forpurifying impure forms of aromatic carboxylic acid.

In another embodiment, the treated second liquid phase is suitable foruse as a seal flush.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the drawings, in which:

FIG. 1 is a flow diagram illustrating an apparatus and process accordingto preferred embodiments of the invention, including integration of theapparatus with other equipment used for manufacture and purification ofaromatic carboxylic acids according to embodiments of the invention; and

FIG. 2 is an expanded view of a preferred form of apparatus according topreferred embodiments of the invention and useful for carrying out theprocess according to embodiments thereof.

DETAILED DESCRIPTION OF THE INVENTION Aromatic Carboxylic Acids

Aromatic carboxylic acids for which the invention is suited includemono- and polycarboxylated species having one or more aromatic rings andwhich can be manufactured by reaction of gaseous and liquid reactants ina liquid phase system. Examples of such aromatic carboxylic acidsinclude terephthalic acid, trimesic acid, trimellitic acid, phthalicacid, isophthalic acid, benzoic acid and naphthalene dicarboxylic acids.The invention is particularly suited for manufacture of pure forms ofterephthalic acid including purified terephthalic acid and so-calledmedium purity terephthalic acids.

Oxidation Step

An oxidation step of the invented process is a liquid phase oxidationthat comprises contacting oxygen gas and a feed material comprising anaromatic hydrocarbon having substituents oxidizable to carboxylic acidgroups in a liquid phase reaction mixture comprising a monocarboxylicacid solvent and water in the presence of a catalyst compositioncomprising at least one heavy metal component. The oxidation step isconducted at elevated temperature and pressure effective to maintain aliquid phase reaction mixture and form a high temperature, high pressurevapor phase. Oxidation of the aromatic feed material in the liquid phaseoxidation step produces aromatic carboxylic acid as well as reactionby-products such as partial or intermediate oxidation products of thearomatic feed material and solvent by-products. The liquid-phaseoxidation step and associated process steps can be conducted as a batchprocess, a continuous process, or a semi-continuous process. Theoxidation step can be conducted in one or more reactors.

Feed Materials

Suitable aromatic feed materials for the oxidation generally comprise anaromatic hydrocarbon substituted at one or more positions, normallycorresponding to the positions of the carboxylic acid groups of thearomatic carboxylic acid being prepared, with at least one group that isoxidizable to a carboxylic acid group. The oxidizable substituent orsubstituents can be alkyl groups, such as a methyl, ethyl or isopropylgroups, or groups already containing oxygen, such as a hydroxyalkyl,formyl or keto group. The substituents can be the same or different. Thearomatic portion of feedstock compounds can be a benzene nucleus or itcan be bi- or polycyclic, such as a naphthalene nucleus. The number ofoxidizable substuents on the aromatic portion of the feedstock compoundcan be equal to the number of sites available on the aromatic portion,but is generally fewer than all such sites, preferably 1 to about 4 andmost preferably 2. Examples of useful feed compounds, which can be usedalone or in combinations, include toluene, ethylbenzene and otheralkyl-substituted benzenes, o-xylene, p-xylene, m-xylene, tolualdehydes,toluic acids, alkyl benzyl alcohols, 1-formyl-4-methylbenzene,1-hydroxymethyl-4-methylben-zene, methylacetophenone,1,2,4-trimethylbenzene, 1-formyl-2,4-dimethyl-benzene,1,2,4,5-tetramethyl-benzene, alkyl-, formyl-, acyl-, andhydroxylmethyl-substituted naphthalenes, such as2,6-dimethylnaphthalene, 2,6-diethylnaphthalene,2,7-dimethylnaphthalene, 2,7-diethylnaphthalene,2-formyl-6-methylnaphthalene, 2-acyl-6-methylnaphthalene,2-methyl-6-ethylnaphthalene and partially oxidized derivatives of theforegoing.

For manufacture of aromatic, carboxylic acids by oxidation of theircorrespondingly substituted aromatic hydrocarbon pre-cursors, e.g.,manufacture of benzoic acid from mono-substituted benzenes, terephthaheacid from para-disubstituted benzenes, phthalic acid fromortho-disubstituted benzenes, and 2.6 or 2.7 naphthalene dicarboxylicacids from, respectively. 2,6- and 2,7-disubstituted naphthalenes, it ispreferred to use relatively pure feed materials, and more preferably,feed materials in which content of the pre-cursor corresponding to thedesired acid is at least about 95 wt. %, and more preferably at least 98wt. % or even higher. A preferred aromatic hydrocarbon feed for use tomanufacture terephthalic acid comprises para-xylene. A preferred feedmaterial for making benzoic acid comprises toluene.

Solvent

Solvent for the liquid phase reaction of aromatic feed material toaromatic carboxylic acid product in the liquid phase oxidation stepcomprises a low molecular weight monocarboxylic acid, which ispreferably a C.sub.1-C.sub.8 monocarboxylic acid, for example aceticacid, propionic acid, butyric acid, valeric acid and benzoic, acid.Lower aliphatic monocarboxylic acids and benzoic acid are preferredbecause they are less reactive to undesirable reaction products thanhigher molecular weight monocarboxylic acids under reaction conditionsused in for liquid phase oxidations to aromatic carboxylic acids and canenhance catalytic effects in the oxidation. Acetic acid is mostpreferred. Solvents in the form of aqueous solutions thereof, forexample about 80 to about 95 wt. % solutions of the acid are most,commonly used in commercial operations. Ethanol and other co-solventmaterials that oxidize to monocarboxylic acids under the liquid phaseoxidation reaction conditions also can be used as is or in combinationwith monocarboxylic acids with good results. When using a solventcomprising a mixture of a monocarboxylic acid and such a co-solvent,co-solvents oxidizable to the same monocarboxylic are preferably used sothat solvent separation steps are not further complicated.

In regard to solvents for the liquid phase oxidation according to theinvention, the expression “solvent monocarboxylic acid” as used hereinin reference to a component of various gaseous or liquid streams refersto a monocarboxylic acid having the same chemical composition as themonocarboxylic acid used as solvent tor the liquid phase oxidation. Suchusage also distinguishes those chemical compositions from othermonocarboxylic acids that may be present as oxidation by-products. Byway of example, when the liquid phase reaction mixture for oxidationincludes acetic acid solvent, the expression “solvent monocarboxylicacid” refers to acetic acid but not other monocarboxylic acid speciessuch as benzoic and toluic acids which are common partial orintermediate oxidation by-products of aromatic feed materials usedaccording to the invention. Also, as will be clear from context, theword “solvent” as used in the expression “solvent monocarboxylic acid”may, but does not necessarily, refer to the function of themonocarboxylic acid to which it refers. Tints, again by way of example,“solvent monocarboxylic acid” described as a component of a liquid phaseoxidation reaction mixture is present as solvent for the mixture;however, “solvent monocarboxylic acid” described as a component presentin a high pressure vapor phase generated in the oxidation or as acomponent of a liquid phase separated from such a vapor phase is notintended to denote that the monocarboxylic acid is functioning as asolvent.

Catalysts

Catalysts used for the liquid oxidation comprise materials that areeffective to catalyze oxidation of the aromatic feed material toaromatic carboxylic acid. Preferred catalysts are soluble in the liquidphase reaction mixture used for oxidation because soluble catalystspromote contact among catalyst, oxygen gas and liquid feed materials;however, heterogeneous catalyst or catalyst components may also be used.Typically, the catalyst comprises at least one heavy metal componentsuch as a metal with atomic weight in the range of about 23 to about178. Examples of suitable heavy metals include cobalt, manganese,vanadium, molybdenum, chromium, iron, nickel, zirconium, cerium or alanthanide metal such as hafnium. Suitable forms of these metalsinclude, for example, acetates, hydroxides, and carbonates. Preferredcatalysts comprise cobalt, manganese, combinations thereof andcombinations with one or more other metals and particularly hafnium,cerium and zirconium.

In preferred embodiments, catalyst compositions for liquid phaseoxidation also comprise a promoter, which promotes oxidation activity ofthe catalyst metal, preferably without generation of undesirable typesor levels of by-products. Promoters that are soluble in the liquidreaction mixture used in oxidation are preferred for promoting contactamong catalyst, promoter and reactants. Halogen compounds are commonlyused as a promoter, for example hydrogen halides, sodium halides,potassium halides, ammonium halides, halogen-substituted hydrocarbons,halogen-substituted carboxylic acids and other halogenated compounds.Preferred promoters comprise at least one bromine source. Suitablebromine sources include bromo-anthracenes, Br₂, HBr, NaBr, KBr, NH₄Br,benzyl-bromide, bromo acetic acid, dibromo acetic acid,tetrabronioethane, ethylene dibromide, bromoacety bromide andcombinations thereof. Other suitable promoters include aldehydes andketones such as acetaldehyde and methyl ethyl ketone.

Oxygen Source

Reactants for the liquid phase reaction of the oxidation step alsoinclude a gas comprising molecular oxygen. Air is conveniently used as asource of oxygen gas. Oxygen-enriched air, pure oxygen and other gaseousmixtures comprising molecular oxygen, typically at levels of at leastabout 10 vol. %, also are useful. As will be appreciated, as molecularoxygen content of the source increases, compressor requirements andhandling of inert gases in reactor off-gases are reduced. When air orother oxygen-containing gaseous mixtures are used as an oxygen sourcefor the process, the high pressure vapor phase generated by the liquidphase reaction in the oxidation step comprises nitrogen or other inertgas components of the oxygen source.

Oxidation

Proportions of aromatic feed material, catalyst, oxygen and solvent arenot critical to the invention and vary with factors that include choiceof reactants, solvent and catalyst compositions and intended aromaticcarboxylic acid product, details of process design and operatingfactors. Solvent to aromatic feedstock weight ratios ranging from about1:1 to about 30:1 are preferred, with about 2:3 to about 5:1 being morepreferred although higher and lower ratios, even in the range ofhundreds to one also can be used. Oxygen gas typically is used in atleast a stoichiometric amount based on aromatic feed material but not sogreat, taking into account reaction conditions, rates and organiccomponents of the high pressure vapor phase resulting from the liquidphase reaction that a flammable mixture exists in the vapor phase. Incommercial operations using preferred aromatic feed materials, solventmonocarboxylic acid, catalyst compositions and operating conditions,oxygen gas, most commonly supplied in the form of air, is preferablysupplied to the liquid phase oxidation at a rate effective to provide atleast about 3 to about 5.6 moles molecular oxygen per mole of aromatichydrocarbon feed material. A high pressure vapor phase resulting fromliquid phase oxidation is preferably removed from the reaction at a ratesuch that oxygen content of the vapor phase in a reaction zone containsfrom about 0.5 to about 8 vol. % oxygen measured on a solvent-freebasis. Other things being equal, variations in vapor phase oxygencontents, such as by increasing or decreasing reaction rates by use ofgreater or lesser amounts of catalyst in the liquid phase oxidation, caninfluence by-product generation in the oxidation, with lower vapor phaseoxygen contents, for example up to about 3 vol. %, or from about 0.5 toabout 2.5 vol. %, tending to favor more complete conversion of aromatichydrocarbon feed to the aromatic carboxylic acid and, in turn, reducedoxidation by-products of the aromatic feedstock but with increasedgeneration of solvent by-products. By way of example, in liquid phaseoxidations using para-xylene feed materials and acetic acid as solventfor oxidation, vapor phase oxygen contents of about 0.5 to about 3 vol,% are preferred for making aromatic carboxylic acid products in whichlevels of para-xylene by-products are reduced but acetic acidby-products are increased as compared to operation at higher vapor phaseoxygen contents. Catalyst suitably is used in concentrations of catalystmetal, based on weight of aromatic hydrocarbon feed and solvent, greaterthan about 100 ppmw, preferably greater than about 500 ppmw, and lessthan about 10,000 ppmw, preferably less than about 6,000 ppmw, morepreferably less than about 3,000 ppmw. Preferably a halogen promoter andmore preferably a promoter comprising bromine, is present. Such apromoter is present in an amount such that the atom ratio of halogen tocatalyst, metal suitably is greater than about 0.1:1, preferably greaterthan about 0.2:1 and suitably is less than about 4:1, preferably lessthan about 3:1. The atom, ratio of halogen to catalyst metal mostpreferably ranges from about 0,25:1 to about 2:1. Other things beingequal, reaction rates and consumption of oxygen gas in liquid phaseoxidation increase and levels of unreacted oxygen in the vapor phasefrom oxidation decrease, with increased catalyst concentrations in theoxidation reaction mixture.

The liquid phase reaction for oxidation of aromatic feed material toproduct comprising aromatic carboxylic acid is conducted in a suitableoxidation reaction zone, which normally comprises one or more oxidationreaction vessels. Suitable oxidation reaction vessels are configured andconstructed to withstand the high temperature and pressure conditionsand corrosive liquid and vapor phase contents used and present in thereaction zone and to provide for addition and mixing of catalyst, liquidand gaseous reactants and solvent, removal of aromatic carboxylic acidproduct or a liquid comprising such product tor recovery thereof, andremoval of a high pressure vapor phase generated by the liquid phasereaction tor controlling heat of reaction. Reactor types which can beused include continuous stirred tank reactors and plug-flow reactors.Commonly, oxidation reactors comprise a columnar vessel, normally with acentral axis which extends vertically when the vessel is positioned forprocess use, having one or more mixing features for mixing liquidreactants and distributing oxygen gas within, the liquid phase boilingreaction mixture. Typically, the mixing feature comprises one or moreimpellers mounted on a rotatable or otherwise movable shaft. Forexample, impellers may extend from a rotatable central vertical shaftReactors may be constructed of materials designed to withstand theparticular temperatures, pressures and reaction compounds used.Generally, suitable oxidation reactors are constructed using inert,corrosion-resistant materials such as titanium or with at least theirsurfaces that define interior space or volume in which liquid reactionmixture and reaction off-gas are contained lined with materials such astitanium or glass.

A reaction mixture for the liquid phase oxidation is formed by combiningcomponents comprising aromatic feed material, solvent and catalyst andadding gaseous oxygen to the mixture. In continuous or semi-continuousprocesses, components preferably are combined in one or more mixingvessels before being introduced to the oxidation zone; however, thereaction mixture can also be formed in the oxidation zone. The source ofoxygen gas can be introduced into the reactor in one or more locationsand is typically introduced in such a manner as to promote contactbetween the molecular oxygen and the other reaction compounds, forexample, by introduction of compressed air or other gaseous oxygensource into the liquid body within a lower or intermediate portion ofthe interior volume of the reaction vessel.

Oxidation of aromatic feed material to product comprising aromaticcarboxylic acid is conducted under oxidation reaction conditionseffective to maintain a liquid phase reaction mixture and form aromaticcarboxylic acid and impurities comprising by-products of the aromatichydrocarbon precursor dissolved or suspended in the liquid phasereaction mixture and generate a high temperature and pressure vaporphase, gaseous components of which are primarily solvent monocarboxylicacid (for example, acetic acid when the oxidation reaction solventincludes acetic acid) and water and, in minor amounts, oxidationby-products of the solvent monocarboxylic acid, such as lower alcoholsand solvent monocarboxylic acid esters thereof (for example, methanoland methyl acetate when the solvent includes acetic acid) and oxidationby-products of the aromatic hydrocarbon feed material such as partialand intermediate oxidation products (for example, benzoic acid andp-toluic acid when the aromatic feed material includes para-xylene).Solvent by-product contents of the vapor phase typically range fromabout 0.5 to about 2 wt. %. Aromatic hydrocarbon precursor by-productlevels are typically about 0.01 to about 0.05 wt. %. The high pressurevapor phase commonly also comprises unreacted aromatic feed material andoxygen, gas that enter the vapor phase. When using air, as commonlypracticed in commercial scale operations, or other oxygen gas sourcescomprising nitrogen or other inert gas components, the vapor phase willalso comprise those inert components. Heat generated by oxidation isdissipated by boiling the liquid phase reaction mixture and removing anoverhead vapor phase from the reaction zone.

Generally temperatures of the liquid phase reaction are maintained atabout 120° C. or greater, and preferably at about 140° C. or greater,but less than about 250° C. and preferably less than about 230° C.Reaction temperatures in the range of about 145° C. to about 230° C. arepreferred in the manufacture of aromatic carboxylic acid products suchas terephthalic acid, benzoic acid and naphthalene dicarboxylic acid. Attemperatures lower than about 120° C., the liquid phase oxidation canproceed at rates or with conversions that are economically unattractiveor may adversely affect product quality. For example, manufacture ofterephthalie acid from para-xylene feedstock at a temperature less thanabout 120° C. can take more than 24 hours to proceed to substantialcompletion and the resulting terephthalie acid product can requireadditional processing due to its impurities content. Temperatures above250° C. are not preferred due to potential for undesirable burning andloss of solvent. Pressure of the liquid phase reaction mixture can beused to control the temperature at which the liquid phase reactionmixture boils and is selected to maintain a substantial liquid phasereaction mixture. Pressures of about 5 to about 40 kg/cm² gauge arepreferred, with preferred pressures for particular processes varyingwith feed and solvent compositions, temperatures and other factors andmore preferably ranging between about 10 to about 30 kg/cm². At areaction pressure of about 7 to about 21 kg/cm², temperature of areaction mixture comprising acetic acid as solvent, and of the vaporphase resulting from the liquid phase reaction, is about 170 to about210° C. Residence times in the reaction vessel can be varied asappropriate for given throughputs and conditions, with about 20 to about150 minutes being generally suited to a range of processes. Formanufacture of some aromatic carboxylic acids, such as manufacture ofterephthalic acid from para-xylene feed materials using acetic acidsolvent for the reaction mixture, solids contents in the boiling liquidphase reaction mixture can be as high as about 50 wt. % of the liquidreaction mixture, with levels of about 10 to about 35 wt. % being morecommon. In processes in which the aromatic acid product is substantiallysoluble in the reaction, solvent, solid concentrations in the liquidbody are negligible. As will be appreciated by persons skilled in themanufacture of aromatic carboxylic acids, preferred conditions andoperating parameters vary with different products and processes and canvary within or even beyond the ranges specified above.

Products of the liquid phase oxidation reaction include aromaticcarboxylic acid oxidized from the aromatic feed material, impuritiescomprising by-products generated as a result of the liquid phasereaction and, as noted above, a high pressure vapor phase that resultsfrom the liquid phase reaction, including boiling of the liquid phasereaction mixture to allow removal of the vapor phase for control ofreaction temperature. Specific examples of by-products of the aromaticfeed material include partial or intermediate oxidation products such astoluic acids, tolualdehydes, carboxybenzaldehydes and hydroxymethylbenzoic acids. By-products of the liquid phase reaction also includesolvent reaction products such as methanol and other lower aliphaticalcohols oxidized from the reaction solvent and esters generated byreaction of such alcohols with the solvent, examples of which includemethyl acetate, methyl propionate, methyl butyrate and the like.By-products commonly are present in one or both the liquid phaseoxidation reaction mixture and the vapor phase resulting therefrom.Carbon oxide by-products can result from oxidation of solvent, feedmaterials or their by-products. In embodiments of the invention in whichthe liquid phase reaction is conducted using a source of bromine aspromoter, by-products also typically include lower alkyl bromides, e.g.,methyl bromide when using acetic acid as the reaction solvent, whichcommonly forms by reaction of bromide ions with acetic acid. As above,these bromine-containing by-products and impurities may be present inone or both of the liquid phase reaction mixture and the high pressurevapor phase generated therefrom, in some embodiments of the inventedprocess, for example those in which solid product from liquid phaseoxidation is purified and a mother liquor or other recycle streamscomprising purification step liquids or components thereof aretransferred directly or indirectly to a liquid phase oxidation or tooff-gas separation as reflux liquid, additional by-products such asbenzoic acid and toluic acids carried over into purification liquids aswell as hydrogenated derivatives of various by-product compoundsresulting from purification steps and unreacted aromatic hydrocarbonfeed to oxidation carried into purification also may be introduced tothe liquid phase oxidation reaction mixture and off-gases.

Water also is produced as a by-product of the liquid phase reaction inthe oxidation step. However, because water may also be present in theliquid phase reaction mixture as a result of addition thereto, forexample when using aqueous monocarboxylic acid solvents or in recyclestreams from other process steps, and also due to the significantamounts of water present in the oxidation step, whether as by-product ordeliberate addition, and inability or lack of need to distinguishbetween water of reaction and water added deliberately, the expression“by-products of the liquid phase reaction” and like expressions usedherein do not refer to water unless stated otherwise. Similarly, whenwater or water vapor is described herein as a component of variousprocess liquids, gases or streams, it is without regard to whether thewater is by-product water from liquid phase oxidation, deliberatelyadded in the process or both unless otherwise stated or clear fromcontext.

Aromatic carboxylic acid reaction product slurried or dissolved in aportion of the liquid reaction mixture from the liquid phase oxidationcan be treated using any suitable techniques for recovering aromaticcarboxylic acid reaction product contained therein. Typically, aromaticcarboxylic acid product and by-products of the aromatic feed material tooxidation slurried, dissolved or slurried and dissolved in liquidreaction mixture are removed from the reaction zone used for the liquidphase reaction and recovered by suitable techniques. Thus, liquid phaseoxidation according to invented process can comprise, in addition to theoxidation reaction, a step comprising recovering from a liquid phaseoxidation reaction mixture a product comprising aromatic, carboxylicacid and impurities comprising reaction by-products. The productpreferably is recovered as a solid product.

Soluble product dissolved in the liquid can be recovered bycrystallization, which usually is accomplished by cooling and releasingpressure on a liquid slurry or solution removed from the oxidationreaction zone. Solid product slurried in the liquid and solidscrystallized from reaction liquid or from crystallization solvents areconveniently separated from the liquids by centrifuging, filtration orcombinations thereof. Solid products recovered from the reaction liquidby such techniques comprise aromatic carboxylic acid and impuritiescomprising by-products of the aromatic feed material. Liquid remainingafter recovery of solid product from the liquid reaction mixture, alsoreferred to as oxidation mother liquor, comprises solvent monocarboxylicacid, water, catalyst and promoter, soluble by-products of the liquidphase oxidation and impurities that may be present such as from recyclestreams. The mother liquor normally also contains minor amounts ofaromatic carboxylic acid and partial or intermediate oxidation productsof the aromatic feed material remaining unrecovered from the liquid. Themother liquor is preferably returned at least in part to the reactionzone of at least one liquid phase oxidation so that components thereofthat are useful in the liquid phase reaction, such as catalyst,promoter, solvent and by-products convertible to the desired aromaticcarboxylic acid, can be re-used.

In preferred embodiments of the invention, a liquid phase reactionmixture from oxidation comprising aromatic carboxylic acid andby-products of a liquid phase oxidation reaction is recovered from theliquid by crystallization in one or more stages, such as in a singlecrystallization vessel or a series of crystallization vessels, withsequential reductions in temperature and pressure from earlier to laterstages to increase product recovery. Crystallization In two to fourstages, for example from an oxidation reaction temperature in the rangeof about 140 to about 250° C. and pressure in the range of about 5 toabout 40 kg/cm² gauge to a final crystallization temperature in therange of about 110 to about 150° C. and pressure of ambient to about 3kg/cm², provides substantial crystallization of solid aromatic acidproduct. Mother liquor separated from the solid product bycrystallization can be returned to the liquid phase reaction asdescribed above, bleat is removed from the vessels used forcrystallization by removal of a gas phase formed as a result of flashingor other pressure letdown of the reaction liquid, with a vapor phaseremoved from one or more stages preferably condensed and, directly orindirectly through one or more additional recovery stages, as discussedbelow, returned at least in part to the reaction zone for use in liquidphase oxidation.

Solid product recovered from the liquid phase oxidation, typicallycomprising aromatic carboxylic acid and impurities comprising oxidationby-products such as intermediate oxidation products of the aromatic feedmaterial, can be separated from liquid oxidation mother liquor resultingfrom recovery of the solid product by any suitable technique. Examplesinclude centrifuging, vacuum filtration, pressure filtration andfiltration using belt filters. The resulting solid product is preferablywashed after separation with liquid comprising water such as pure wateror a wash liquid comprising minor amounts of solvent monocarboxylicacid, catalyst, aromatic feedstock, oxidation by-products orcombinations thereof that can be beneficially recycled to oxidation,either directly or combined with other liquids such as oxidation motherliquor recycle or other liquids returned to the reaction zone.Separation of solid impure aromatic carboxylic acid recovered from anoxidation mother liquor and washing of solid product can be convenientlyaccomplished by solvent exchange filtration under pressure usingpressure filters such as are disclosed in U.S. Pat. Nos. 5,679,846, and5,200,557. A preferred filtration device for such separations is a BHSFest filter as described more fully in U.S. Pat. No, 5,200,557. Motherliquor and wash liquids removed from the filtered cake can betransferred directly or indirectly to liquid phase oxidation. Filtrationand washing of the solid product in multiple stages and withincreasingly pure wash liquids, for example liquids removed from filtercake in downstream stages as wash liquid in prior stages, can provideadditional benefit by concentrating solvent monocarboxylic aciddisplaced from filtered solids tor return to oxidation. In a morespecific embodiment, the filtered cake wet with wash liquid resultingfrom such positive displacement filtration is directed from a final washstage to a drying stage wherein it is optionally contacted with inertgas, typically under light to moderate pressure, for substantial removalof residual liquid from the cake. After washing and substantial removalof wash liquid from solid product comprising aromatic acid andby-products, the resulting solid can be dried and directed to storage orother steps, which may include preparation of a reaction solution forpurification, of the solid product. Preferably, levels of residualsolvent monocarboxylic acid in solid product directed to purificationare about 5,000 parts per million by weight (“ppmw”) or less. Solidproduct can be dried with a flowing stream of nitrogen or other inertgas to reduce residual solvent levels.

In addition to the aromatic carboxylic acid reaction product formed inthe liquid phase reaction of an oxidation step according to the inventedprocess, a high pressure vapor phase is generated, comprising solventmonocarboxylic acid, water and by-products of the liquid phaseoxidation, as described above. The vapor phase commonly also containsminor amounts of unreacted aromatic feed material, unconsumed oxygen gasand, if present, inert components of the oxygen source. Temperature andpressure of the vapor phase present in the reaction zone corresponds toconditions of the liquid phase reaction. An off-gas separation accordingto the invention provides for recoveries of materials and in someembodiments, energy and combinations thereof from the high temperatureand pressure off-gas removed from a liquid phase oxidation reaction.

Separation of Monocarboxylic Acid-Rich First Liquid Phase and Water-RichSecond Liquid Phase

An off-gas separation according to the invented process comprisestransferring the vapor phase removed from the reaction zone of a liquidphase oxidation to a separation zone capable of substantially separatingsolvent monocarboxylic acid, water and oxidation by-products into atleast one solvent monocarboxylic acid-rich first liquid phase and atleast one water-rich second liquid phase that is substantially free ofsolvent monocarboxylic acid and at least one solvent monocarboxylicacid-depleted second high pressure vapor phase comprising water vaporsuch that oxidation by-products of the aromatic hydrocarbon precursorare preferentially apportioned to the first liquid phase and oxidationby-products of the solvent monocarboxylic acid are preferentiallyapportioned to the second high pressure vapor phase. A solventmonocarboxylic acid-rich first liquid phase and a water-rich secondliquid phase that is substantially free of solvent monocarboxylic acidand oxidation by-products thereof and a second high pressure vapor phasethat is substantially free of oxidation by-products of the aromatichydrocarbon precursor are removed from the separation zone. Separationis conducted with the high pressure vapor phase at a temperature andunder pressure not substantially less than temperature and pressure ofthe vapor phase in the liquid phase oxidation step from which the vaporphase is removed.

In greater detail, separation comprises directing a high pressure andtemperature vapor phase removed from the reaction vessel used for liquidphase oxidation to a separation zone that is capable of operating withthe vapor phase at high temperature and pressure to substantiallyseparate water and solvent monocarboxylic acid in the vapor phase andapportion by-products from the oxidation among liquid and gas phasesresulting from the separation such that solvent by-product content ofthe liquid phases and aromatic hydrocarbon oxidation by-product contentof a gas phase removed from separation are minimized. The high pressurevapor phase can be transferred from the reaction zone of a liquid phaseoxidation to the separation zone directly, as where a separation deviceis mounted directly or in close association with an oxidation reactionvessel or other reaction zone, or indirectly, for example by meanssuitable conduits, valves, pumps and the like for effecting transfer. Aminor portion of the high pressure and high temperature vapor phase fromthe liquid phase oxidation may be directed to other uses, such asgeneration of high pressure steam or heat exchange fluid. Preferably,the vapor phase transferred to separation remains at high enoughtemperature and pressure so that energy content of the vapor phaseentering the separation zone is at least substantially retained and thevapor phase provides sufficient heat for separation in contact withreflux liquid supplied to the separation zone. Most preferably, transferof the vapor phase to the separation zone is achieved by passagedirectly from the reaction zone or through suitable pressure ratedpiping such that temperature of the vapor phase entering the separationzone is no more than about 10° C. cooler than the reaction temperaturein the liquid, phase oxidation and pressure of the vapor phase enteringthe separation zone is no more than about 3 kg/cm² less than thepressure in the liquid phase oxidation. The separation zone also isdesigned for operation at high temperature and pressure, and preferablyat temperatures and pressures not substantially less than thetemperature and pressure of the high pressure vapor phase present in thereaction zone to avoid loss of energy content of the vapor phase fromthe reaction zone. More preferably, the separation zone is designed fortreating a vapor phase under pressure of at least about 80%, morepreferably at least about 90%, and still more preferably at least about95%, of the pressure of the vapor phase in the oxidation step. Pressurerating of equipment of the separation zone preferably is at least about80%, more preferably about 90 to about 110%, of the rating of theoxidation reaction vessel or zone of the oxidation step of the inventedprocess from which the vapor phase is directed to separation.Temperatures of the vapor phase in the separation zone preferably rangefrom about 140 to about 200° C. and more preferably from about 160 toabout 185° C. Pressures from about 5 to about 40 kg/cm² are preferred,with about 10 to about 20 kg/cm² being more preferred.

The separation zone is capable of substantially separating solventmonocarboxylic acid and water vapors in the high pressure vapor phaseintroduced to separation. Preferably the separation zone is capable ofseparating water and solvent in the high pressure vapor phase such thata high pressure gas resulting from the separation contains no more thanabout 10%, and more preferably no more than about 5% of the solventmonocarboxylic acid content of the vapor phase introduced to theseparation zone. More preferably, solvent monocarboxylic acid content ofa second high pressure gaseous effluent from separation is no more thanabout 2%, and still more preferably no more than about 1%, of thesolvent monocarboxylic acid content of the vapor phase introduced to theseparation zone. The separation zone is also adapted for preferentiallyapportioning to at least one liquid phase by-products of the aromaticfeed material to oxidation and to the second high pressure vapor phaseby-products of the solvent monocarboxylic acid that, otherwise apportionnormally to both vapor and liquid phases at the temperatures andpressures at which the separation is conducted. For example, in the caseof liquid phase oxidation of para-xylene feed materials in a liquidphase reaction mixture comprising acetic acid solvent, benzoic acid andp-toluic acid by-products of the para-xylene and methanol and methylacetate by-products of the acetic acid can apportion at practicallysignificant levels between vapor and liquid phases. The separation zoneis capable of apportioning by-products such that the second highpressure vapor phase is substantially free of by-products of thearomatic hydrocarbon precursor and preferably contains no more thanabout 10 wt. %, and more preferably about 1 to about 5 wt. % thereof.By-products of the aromatic hydrocarbon precursor removed to die first,solvent monocarboxylic acid-enriched liquid phase and the second,water-enriched liquid phase are preferably apportioned preferentially tothe first phase, and more preferably such that about 75 wt %, still morepreferably at least about 85 wt. %, to about 100 wt. % thereof, arepresent in the first liquid phase and no more than about 25 wt, %, stillmore preferably no more than about 2 to about 10 wt. %, thereof arepresent in the second liquid phase. By-products of the solventmonocarboxylic acid comprising alcohols and solvent acid esters thereofare preferably apportioned to the second high pressure vapor phaseresulting from separation of water and solvent monocarboxylic acid inthe inlet high pressure vapor phase, preferably such that such that thesecond, water enriched, liquid phase contains no more than about 10 wt.%, and more preferably no more than about 1 to about 4 wt. % of suchby-products.

The separation zone for off-gas separation according to the inventioncan comprise any device or means suitable for substantially separatingsolvent monocarboxylic acid and water in the high temperature andpressure vapor phase removed from the liquid phase oxidation andapportioning oxidation by-products present in the device at hightemperature and pressure to obtain a liquid phase rich in solventmonocarboxylic acid, a second liquid phase enriched in water and asecond high pressure vapor phase comprising water, as described above.

In one embodiment, a preferred separation zone is adapted for contactbetween vapor and refluxing liquid phases flowing countercurrentlytherethrough such that solvent monocarboxylic acid in the high pressurevapor phase introduced to the separation zone from a liquid phasereaction zone is substantially removed from the vapor phase to theliquid phase to form a first liquid phase which is enriched in thesolvent monocarboxylic acid, and such that water from a resultingsolvent monocarboxylic acid-depleted high pressure vapor phase isremoved into the refluxing liquid phase for withdrawal from theseparation zone of a second liquid phase which is enriched in water.Oxidation by-products of the aromatic feed to liquid phase oxidationthat tend to apportion between both the vapor and liquid phases underconditions in the separation are present in the high pressure vaporphase introduced to the separation zone from a liquid phase oxidationand may also be introduced into the separation zone in reflux liquidssupplied thereto. Such by-products apportioned to the liquid phase towhich solvent monocarboxylic acid from the high pressure vapor phasefrom oxidation is removed can be removed in the first liquid phase. Suchby-products present in the solvent monocarboxylic acid-depleted vaporphase are further apportioned to that liquid phase and also enter theliquid phase to which water from the solvent-depleted vapor phase isremoved due to contact with the refluxing liquid phase. By-products ofthe solvent monocarboxylic acid that tend to apportion between vapor andliquid phases can be present in the high pressure vapor phase fromoxidation introduced to the separation zone. They also can be present inreflux liquids supplied to the separation zone. Such by-products presentin the refluxing liquid phase in the separation device are stripped fromthe vapor phase by the refluxing liquid.

The flow of refluxing liquid in such a separation device comprisesliquid components removed or apportioned from the vapor phase to theliquid phase as well as components of reflux liquids supplied to theseparation zone that are or remain in the liquid phase.

A preferred separation zone according to a more specific embodiment ofthe invention is configured for stagewise contact between liquid andvapor phases in countercurrent flow through portions or regions of theseparation zone. The vapor phase flow is preferably an ascending flowthrough the portions of the separation zone and the liquid phase flowpreferably is a descending flow therethrough. Separation of water,solvent monocarboxylic acid and by-products is accomplished by directingthe high pressure vapor phase removed from the reaction zone to a firstportion of the separation, zone and a reflux liquid to a third portionof the separation zone such that a vapor phase flow through the firstportion to a second portion to a third portion of the separation zone isin contact with a countercurrent flow of refluxing liquid phase throughthe third to the second to the first portion of the separation zone.Reflux liquid supplied to the third portion comprises water andpreferably is substantially free of oxidation by-products of thearomatic feed materials for liquid phase oxidation. Water and solventmonocarboxylic acid in the countercurrently flowing vapor phase andrefluxing liquid phase are substantially separated in the first portionsuch that a solvent monocarboxylic acid-rich first liquid phase and ahigh pressure, solvent monocarboxylic acid-depleted intermediate vaporphase are formed. The solvent-rich first liquid phase from the firstportion is collected for removal from the separation zone. The vaporphase flow from the first to the second portion of the separation deviceincludes the intermediate vapor phase from the first portion. Water andby-products in the countercurrently flowing vapor phase and refluxingliquid phase in the second portion are separated such that by-productsof the aromatic hydrocarbon precursor are removed to the refluxingliquid phase and a high pressure second intermediate vapor phasecomprising water vapor substantially free of solvent monocarboxylic acidand by-products of the aromatic hydrocarbon precursor is formed. Theflow of vapor phase from the second to the third portions of theseparation zone includes the second intermediate vapor phase. Water andby-products of the solvent monocarboxylic acid in the countercurrentlyflowing vapor phase and refluxing liquid phase in. the third portion areseparated such that a water-enriched second liquid phase substantiallyfree of solvent monocarboxylic acid and by-products thereof and a secondhigh pressure vapor phase comprising water vapor and by-products of thesolvent monocarboxylic acid and substantially free of by-products of thearomatic hydrocarbon precursor are formed. The water-enriched secondliquid phase from the third portion is collected for withdrawal from theseparation device in a liquid stream separate from that in which thefirst liquid phase is withdrawn. The second high pressure vapor phase isremoved from the separation device as an exit gas. The flow of refluxingliquid phase through the separation zone can be supplemented bysupplying additional reflux liquid comprising water to one or moreportions of the separation zone. In preferred embodiments, liquidcomprising water is supplied as additional reflux between the second andthird portions of such a separation zone.

In such a staged separation, the first portion of the separation zonepreferably is capable of separating solvent monocarboxylic acid andwater such that at least 95 wt. %, and more preferably at least about 98wt. %, of the solvent is removed to the first liquid phase. The secondportion is preferably capable of apportioning by-products of thearomatic precursor for the liquid phase oxidation to the first andsecond liquid phases such that the second high pressure vapor phasecontains no more than about 10%, and more preferably about 1 to about5%, of the amount of such by-products present in the first and secondliquid phases and the second high pressure vapor phase. The thirdportion of the separation zone preferably is capable of apportioningliquid phase oxidation by-products of the solvent monocarboxylic acid tothe second high pressure vapor phase such that the second liquid phasecontains no more than about 10%, and more preferably about 1 to about4%, of the amount of such by-products present in the first and secondliquid phases and the second high pressure vapor phase.

In preferred embodiments, a first portion of the separation zone isdefined as a region of the separation zone located between an inlet forreceiving high pressure vapor phase removed from a liquid phaseoxidation into the separation zone and an inlet for introducing a liquidcomprising water to the separation zone as reflux. A second portion ofthe separation zone is defined by a region of the zone positionedbetween an inlet for introducing liquid comprising water as reflux tothe first portion and an outlet for removing the water-rich secondliquid phase collected from the third portion. The third portion isdefined as a region between an outlet for removing water-enriched secondliquid phase collected from the third portion and an inlet forintroducing liquid comprising water substantially free of oxidationby-products of the aromatic feed material for liquid phase oxidation tothe separation device.

According to embodiments of the invention, a separation zone for theseparation and preferential apportionment of water, solventmonocarboxylic acid and by-products comprises a fractionating zonehaving at least about 20 theoretical equilibrium stages forsubstantially separating water and solvent monocarboxylic acid in thehigh pressure vapor from liquid phase oxidation. More preferably, such afractionating zone has about 20 to about 60 theoretical equilibriumstages. A fractionating zone with at least about 2 theoreticalequilibrium stages Is preferred for separating water and oxidationby-products of the aromatic feed material More preferably, such afractionating zone provides about 2 to about 10 theoretical equilibriumstages, A fractionating zone for separating water and oxidationby-products of the solvent monocarboxylic acid preferably has at leastone, and more preferably about 1 to about 10 theoretical equilibriumstages.

Preferred separation devices are various columns or towers, oftenreferred to as distillation columns and towers, dehydration towers,rectifying columns, water removal columns and high efficiency separationdevices, that are designed for contact between gas and liquid phasesflowing therethrough for mass transfer between the phases in a pluralityof theoretical equilibrium stages, also sometimes referred to as“theoretical plates,” configured for separating and preferentiallyapportioning components of the flowing gas and liquid phases. Contactbetween flowing gas and liquid phases is promoted by internal structure,such as trays or packing providing surfaces for gas-liquid contact andtheoretical equilibrium stages for separations. Temperature of the highpressure vapor phase removed from oxidation normally is high enough thatthere is no need for reboiling capability beyond that provided by theliquid phase oxidation reaction. Countercurrent flow of gas and liquidphases, such as by introducing the high pressure vapor phase fromoxidation at a lower portion of the device and reflux liquid at leastone, and preferably two or more upper portions, is preferred forpromoting contact between gas and liquid phases in the separationdevice.

The separation zone according to the invention can comprise a singledevice or multiple devices, such as towers, columns or other structure,in series. When using two or more devices in series, they areconfigured, and their respective inlets and outlets communicate suchthat high pressure vapor phase removed from an oxidation reaction vesselflows into the series with separation and apportionment of solventmonocarboxylic acid, water and by-products in flowing vapor and reverseflows of refluxing liquid in and through the series.

Reflux liquid supplied to the separation zone comprises water. Anysuitable source of liquid comprising water and substantially free ofimpurities detrimental to separation can be utilized. Demineralizedwater or other purified sources can be used but preferred sources ofreflux liquid include liquids condensed from high pressure gases removedfrom separation and/or condensing zones according to the inventedprocess, in another preferred embodiment, purification mother liquorobtained in recovery of a purified aromatic carboxylic acid product fromat least one purification liquid reaction mixture is directed toseparation such that reflux to the separation comprises the purificationmother liquor. Most preferably, reflux liquid for separation comprisessuch a purification mother liquor and liquid comprising water condensedfrom high pressure gases removed from a separation zone, which may besupplied to separation Individually or combined in one or moreindividual streams.

In a staged separation according to preferred embodiments of theinvention as described above, reflux liquid comprising purificationmother liquor is introduced to the separation zone for flow of liquidphase components thereof through the second portion of the zone andcondensate liquid recovered from a second high pressure vapor phaseremoved from the separation zone is introduced for flow through thethird portion. The purification mother liquor typically containsby-products of the aromatic hydrocarbon feed material to liquid phaseoxidation, including hydrogens ted derivatives thereof resulting frompurification, but such by-products are preferentially apportioned to theliquid phases recovered in separation, and predominantly to a solventmonocarboxylic acid-rich first liquid phase, which is suitable forreturn to oxidation to provide make-up solvent. Liquid comprising watercondensed from the second high pressure vapor phase removed from theseparation zone is substantially free of by-products of the aromaticfeed material but can contain by-products of the solvent monocarboxylicacid stripped into the second high pressure vapor phase in separationwhich, in turn, may be present in liquid comprising water condensed fromthe second high pressure gas. Such by-products returned to separation inreflux liquid supplied to a third portion of the separation zone arestripped back into the second high pressure vapor phase in separation.Undesirable accumulation of such by-products is prevented in preferredembodiments of the invention in which a portion of the condensate liquidrecovered from the second high pressure vapor phase from separation ispurged or directed to treatment for recovery of such by-products.

Reflux liquid preferably is supplied at a rate and temperature effectiveto quench heat of the liquid phase oxidation reaction transferred to theseparation zone in the vapor phase from the oxidation. When theseparation zone is coupled to a reaction vessel from liquid phaseoxidation for substantially direct transfer of vapor phase fromoxidation to separation, the reaction vessel functions as a reboiler. Insuch embodiments, the rate at which liquid reflux is supplied to theseparation zone is conveniently expressed as weight of liquid providedto the zone relative to weight of aromatic feed material introduced tothe liquid phase oxidation. Preferably, reflux liquid provided to theseparation zone according to the invented process is at a temperature inthe range of about 120 to about 170° C. and more preferably at about 130to about 160° C. At such temperatures, liquid preferably is supplied toseparation at a rate of about 4 to about 5 weights of the liquid perweight of aromatic precursor introduced to the liquid phase oxidation.When reflux liquid is supplied separately to different stages of aseparation zone it preferably is apportioned between the differentstages such that reflux supplied to a first stage of the separation zonemakes up at least 40%, and more preferably about 60 to about 90%, of thevolumetric flow of reflux liquid.

Water and solvent monocarboxylic acid vapors contained in the highpressure vapor stream removed from a liquid phase oxidation step andintroduced into the separation zone are separated such that a solventmonocarboxylic acid-rich first liquid phase which is lean in water isrecovered. The separated first liquid phase preferably comprises atleast about 60 wt. % solvent monocarboxylic acid and no more than about35 wt % water. More preferably, water content of the separated liquidphase is about 15 to about 30 wt %. The liquid stream from separationalso contains minor amounts of heavier impurities, such as partial orintermediate oxidation by-products of the aromatic feed material andhydrogenated derivatives thereof, such as benzoic acid and, depending onaromatic precursor used in the oxidation, m-toluic acid and/or p-toluicacid, washed or transferred into the first liquid phase in theseparation zone. The first liquid phase may also include othercomponents, such as aromatic carboxylic acid product and catalystmetals. Content of such heavier components can be as high as about 2 wt.% but preferably is no more than about 0.5 wt. %.

The solvent monocarboxylic acid-rich liquid phase condensed from thevapor phase in the separation zone is a valuable source of solvent forliquid phase oxidation. As described above, it also may includeoxidation by-products of the aromatic feed material and other componentssuitable for being returned to oxidation and converted to the desiredaromatic carboxylic acid. Other suitable uses for the liquid condensateinclude wash liquids for rotary vacuum filters or other devices used forsolid-liquid separations of recovered solid products of a liquid phaseoxidation from oxidation mother liquors or crystallization solvents andmake up to scrubbers, such as oxidation dryer scrubbers if used in theprocess. In a preferred embodiment of the invented process, at least aportion, and, more preferably, all or substantially ail of the separatedfirst liquid phase removed from the separation zone is returned toliquid phase oxidation, either directly to a reaction vessel or toholding vessels used for supply of makeup solvent to a reaction zone. Insuch embodiments, water and solvent monocarboxylic acid in the highpressure vapor phase introduced to the separation zone are preferablyseparated such that a liquid phase resulting from the separationcontains about 15 to about 30 wt. % water and, more preferably, suchthat water content of the separated liquid together with water returnedto oxidation in other liquid streams from the process are substantiallybalanced with water vapor removed from oxidation in the high pressureoverhead vapor phase and liquid water removed front oxidation forrecovery and separation of aromatic carboxylic acid product of theoxidation.

The second liquid phase recovered from separation is enriched in waterand substantially free of solvent monocarboxylic acid by-productsthereof from liquid phase oxidation, it may contain minor amounts ofby-products of the aromatic feed material to the liquid phase oxidationas a result of preferential apportionment of such by-products to liquidphases in separations according to the invention. Solvent monocarboxylicacid content of the second liquid phase typically is less than about 5wt. % and preferably about ½ to about 3 wt. %. Solvent by-product levelstypically are no greater than about 1 wt. % and preferably about 0.05 toabout 0.2 wt. %. By-products of aromatic feed materials present in thesecond liquid phase typically range from about 0.003 to about 0.1 wt. %and preferably about 0.005 to about 0.05 wt. %. The second liquid phasefurther comprises dissolved O₂ and gaseous methyl bromide wherein O₂ ispresent in an amount of about 2.2 ppmw and methyl bromide is present inan amount of about 0.03 ppmw.

It has been discovered however that use of the recovered second liquidphase in other parts of the process prior to further treatment to removethe dissolved O₂ and methyl bromide can cause corrosion. Therefore thesecond liquid phase must undergo further treatment to reduce the amountof corrosive compounds residing or dissolved therein to reduce itscorrosive effects. Surprisingly, it has been found that flashing thesecond liquid phase as it exits the separation zone significantlyreduces the amount of dissolved O₂ and methyl bromide dissolved in thetreated second liquid phase to wherein the amount of O₂ is less thanabout 1.0 ppmw, and more preferably the amount of O₂ is less than about0.5 ppmw, and even more preferably the amount of O₂ is less than about0.05 ppmw, and most preferably the amount of O₂ is less than about 0.006ppmw. and the amount of methyl bromide is less than about 0.02 ppmw, andmore preferably the amount of methyl bromide is less than about 0.01,and even more preferably the amount of methyl bromide is less than about0.009, and most preferably the amount of methyl bromide is less thanabout 0.006 ppmw after flashing. As used herein the term “flashing” or“flashed” refers to the immediate drop in pressure ranging from about 5kg/cm² to about 40 kg/cm² in the separation zone to atmospheric orambient pressure in the flash tank, or more preferably from about 10kg/cm² to about 20 kg/cm² in the separation zone to atmospheric orambient pressure in the flash tank that the second liquid phaseexperiences as it exits the separation zone into a flash tank.

Another method of treating the second liquid phase to reduce oreliminate the dissolved O₂ and methyl bromide is steam stripping thesecond liquid phase after it has exited the separation zone and is in aflash tank at atmospheric or ambient pressure. Yet another method oftreating the second liquid phase to reduce or eliminate the dissolved O₂and methyl bromide is to use gaseous nitrogen to strip the second liquidphase after it has exited the separation zone and is in a flash tank atatmospheric or ambient pressure. In yet another embodiment, the secondliquid phase is stopped with steam or nitrogen while under a pressure offrom about 5 to about 40 kg/cm² and more preferably under a pressure offrom about 10 to about 20 kg/cm².

The treated second liquid phase is suitable for use as liquid comprisingwater in one or more steps of a process for purifying impure forms ofaromatic carboxylic acid as described more fully herein. Other uses forthe treated second liquid phase include seal flush liquids tosolid-liquid separation devices used for separating oxidation motherliquor and wash, liquids from impure solid aromatic carboxylic acidproduct recovered from a liquid phase oxidation reaction mixture.

The Second High Pressure Gas

The second high pressure gas resulting from separation comprises asubstantial volume of water and is relatively free of solventmonocarboxylic acid. Preferably the gas comprises at least about 55 vol.%, and more preferably at least about 65 vol. %, water. Solventmonocarboxylic acid content of the gas is generally less than about 5and preferably less than about 3 wt %. The gas also may contain,unreacted aromatic feed material and by-products of the liquid phaseoxidation although they typically are present in minor or trace amountsno greater than about 2 wt %. Oxygen gas content of the pressurized gasfrom separation typically ranges up to about 4 vol. %, preferably fromabout 1 to about 4 vol. %. Inert gas components of the oxygen source,which typically include nitrogen and carbon oxides, can constitute up toabout 45 vol. % of the pressurized gas; when using air as a gaseousoxygen source, nitrogen content of the pressurized gas typically rangesfrom about 30 to about 40 vol %.

Preferably, pressure of the second high pressure vapor phase from theseparation is up to about 1 kg/cm² gauge less than the pressure in theliquid phase oxidation reaction. Temperature of the high pressure gasfrom separation is preferably up to about 20° C. less than thetemperature of the liquid phase oxidation reaction, and more preferablyabout 5° C. to about 15° C. less than the oxidation reactiontemperature. Preferably, the high pressure gas from the separation is ata temperature greater than about 100° C., more preferably greater thanabout 120° C., and less than about 250° C., more preferably less thanabout 230° C. Pressure of the pressurized gas remaining after theseparation is about 4 to about 40 kg/cm² gauge.

The second high pressure vapor phase removed from separation can bedirected to a condensing zone for condensing from die vapor phase aliquid condensate comprising water substantially free of organicimpurities such as solvent monocarboxylic acid and by-products of thearomatic feed material and solvent from oxidation. The condensing zonecan comprise any means effective for condensing water substantially freeof organic impurities from the high pressure gas introduced to thecondensing zone. Preferably, it includes one or more condenser or heatexchange means effective for providing indirect heat transfer between ahigh pressure gas phase and a heat sink material, and preferably a heatexchange fluid. A single device or a plurality of devices in series canbe employed. Shell and tube heat exchangers and kettle type condensersare examples of preferred devices. Preferably, ail or substantially allof the high pressure vapor from separation is directed to the condensingzone to enable substantial recovery of both heat energy and materialstherefrom. Cooling preferably is conducted under conditions such that acondensing zone exhaust gas under pressure not substantially reducedfrom that of the gas introduced to the condensing zone remains aftercondensing the liquid condensate and is withdrawn from the condensingmeans. That pressurized condensing zone exhaust gas comprisesincondensable components of the high pressurize gas from the separationzone, gaseous reaction by-products and may also contain minor amounts ofaromatic feed material from the liquid phase oxidation off-gas or refluxliquids directed to separation and remaining unseparated in the secondhigh pressure vapor phase. The exhaust gas from, the condensing zonemost preferably is at a temperature of about 50 to about 150° C. andunder pressure that is no more than about 3 kg/cm² less than thepressure of the inlet gas to the condensing zone. More preferably, thepressure differential between a gas removed from the separation deviceand the condensing zone exhaust gas after condensation of liquidcondensate is about 2 kg/cm² or less and most preferably about 0.5 toabout 1 kg/cm^(2.)

Cooling of high pressure gas by heat exchange with a heat sink materialin the condensing zone also serves to heat the heat sink material. Theheat sink material preferably is a heat sink fluid, and most preferablywater. When using water as the heat exchange fluid, heat exchange withthe high pressure gas from separation converts the water to steam whichcan he directed to other parts of the invented process for heating or touses outside the process. Similarly, heat exchange between thepressurized gas and liquids from other process steps can be used forheating such liquids. Thus the invention includes embodiments in whichheat exchange between the high pressure gas from the separation zoneintroduced to the condensing zone and heat exchange fluid comprisingwater is conducted in a series of heat exchangers operated atsuccessively cooler temperatures such that steam at different pressuresis generated from the heat exchange water. Steam at different pressuresis preferably directed to one or more process steps in which steam undercorresponding pressure or pressures is useful for heating, while liquidcondensate comprising water at successively lower temperatures isgenerated from the pressurized gas.

Energy can be recovered from exhaust gas from the condensing zone in theform of heat, in the form of work or as both. Recovering energy as heatfor the process can reduce consumption of fuel that would otherwise beneeded to generate heat for the process. Energy recovered as work can beconverted to electricity for use in the process, thereby reducingconsumption of electricity from external sources if used in the process.

While preferred embodiments of the invention comprise condensing ah orsubstantially all of the high pressure gas transferred to the condensingzone, in some embodiments of the invention, condensation of highpressure gas removed from the separation zone is conducted by extractingheat energy from the gas such that only a portion of the water contentof the gas is condensed or by directing a portion of the second highpressure vapor phase from separation to condensing means and directinganother portion to means for recovery of energy by conversion tomechanical energy. Partial condensation of the second high pressurevapor phase removed from separation or splitting the stream forcondensation of only a portion thereof allows recovery of a liquidcondensate comprising substantially pure water with low organicimpurities content and useful as reflux liquid for separation asdescribed above and recovery of heat energy transferred to a heatexchange fluid on cooling of the high pressure gas to condense theliquid condensate, while also leaving uncondensed water in a highpressure condensing zone exhaust gas for further energy recovery in theform of work.

According to other embodiments of the invention, ail or substantiallyall of the second high pressure vapor phase from separation ofmonocarboxylic acid and water in the high pressure vapor phase fromoxidation and oxidation by-products is condensed by heat exchange with aheat sink fluid. Condensation of ail or substantially ail of thecondensable components of the high pressure gas from, separation reducesthe volumetric flow of gas remaining after condensation to subsequentprocessing steps and permits use of metals with only low or moderatecorrosion resistance, such as stainless steels, mild steels or duplexsteels, as alternatives to more expensive, highly corrosion resistantmetals or alloys in equipment for subsequent off-gas treatment stepsthat may be included in the process. Substantially complete condensationof condensable components of a high pressure gas removed from separationalso increases the volume of liquid condensate comprising watersubstantially free of organic impurities generated according to theinvented process and can facilitate enhanced recovery of aromatic feedmaterial and solvent monocarboxylic acid or liquid phase oxidationby-products thereof remaining in uncondensed gases remaining aftercondensation.

Condensation can be conducted in a single step. It also can be conductedin multiple steps in which a gas stream comprising high pressure gasremoved from a separation zone is cooled to a first temperature in afirst stage to yield a first stage condensate liquid and an uncondensedportion of the gas which is subsequently condensed at a lowertemperature in a second stage to provide a second stage condensateliquid and an uncondensed portion of the gas introduced to the secondstage, and optionally one or more additional stages in which anuncondensed portion of gas from a prior stage is condensed at a lowertemperature than in the previous stage to form a liquid condensate and aremaining uncondensed gaseous portion. Heat exchange between thepressurized gas and uncondensed portions thereof in the stagedcondensers provides heat exchange fluid at different temperatures orpressures, for example moderate and low pressure steam, which can beused for heating in other process steps or outside the process. Inpreferred embodiments of die invention, two or more levels of steam areproduced for energy recovery, which is conveniently accomplished using acondensing or other low pressure steam turbine. In such embodiments,condensate liquid removed at different temperatures can be directed toother process uses with corresponding temperatures, thereby avoidingadditional heating or cooling of the condensate portions and, in somecases, limiting buildup of impurities such as solvent monocarboxylicacid oxidation by-products in steps to which condensate liquids arerecycled. For example, condensate liquids recovered at highertemperatures, for example in the range of about 130 to about 160° C.,are well suited, with little or no additional heat input, as reflux toseparation as such or in combination with aqueous liquids from otherprocess steps such as mother liquor remaining after recovery and/orseparation of purified aromatic carboxylic acid in a purification step.Such high temperature condensate liquids can provide additional benefitwhen used as reflux to separation due to their lower content of lightcomponents, such as lower alcohols and solvent monocarboxylic acidesters thereof that are generated as solvent by-products in liquid phaseoxidation and tend to condense in greater concentrations in lowertemperature condensate liquids. Lower temperature condensates, forexample those in the range of about 60 to about 90° C., are also wellsuited for hot condensate uses such as wash liquids for productseparations and seal flush liquids in liquid phase oxidation,purification or both, and still cooler condensate, for example in therange of about 40 to about 50° C., for cold condensate uses such asscrubber washes. While condensation at different temperatures such thatcondensate liquid can be directed to other process uses with compatibletemperatures provides options for favorable energy management in theinvented process, it will be appreciated that liquid condensate portionsor streams condensed at higher or lower temperatures than may be neededor preferred for use in other steps can be cooled or heated as may bedesired, for example by heat exchange, for use in such other steps.

Exhaust gas from the condensing zone preferably is under pressure and,while substantially free of water vapor according to preferredembodiments of the invention, can retain a portion of the water from thesecond high pressure vapor phase from separation depending on the extentof condensation in the condensation step. In addition to such watervapor as may be present in the exhaust gas, the gas can compriseincondensable components from the liquid phase oxidation off-gas, suchas unreacted oxygen from oxidation, nitrogen, carbon oxides and otherinert gas components if present in the oxygen source for oxidation, andcarbon oxides, and may contain minor amounts of solvent monocarboxylicacid by-products from oxidation and traces of solvent monocarboxylicacid, other oxidation by-products and unreacted aromatic hydrocarbonfeed material not removed in other steps. Even when water in the exhaustgas is substantially completely condensed into the liquid condensate,such that the uncondensed exhaust gas remaining after condensation issubstantially free of water, pressure of the exhaust gas is alsosufficiently high and, especially when the gaseous oxygen source forliquid phase oxidation is air or another gaseous mixture withsignificant inert gas content such that the vapor phase removed fromoxidation and, in turn, pressurized gases from the separation andcondensing zones contain substantial inert gas content, volume of thecondensing zone exhaust gas is such that it can be a useful source forrecovery of energy.

According to embodiments of the invention, energy can be recovered fromthe pressurized exhaust gas from condensation. Preferably, energy isrecovered in the form of work. In these embodiments, a pressurized gasstream comprising exhaust gas from the condensing zone is transferred,directly or indirectly, to a device for recovering energy as work, Apreferred energy recovery device is an expander or similar apparatusadapted to receive a flow of gas under pressure and equipped with bladescapable of being rotated by the flowing gas, thereby generating workuseful in other process steps or outside the process and a cooled gasunder reduced pressure. Work extracted from the pressurized gas can beused, for example, to generate electricity using a generator or foroperating a compressor used to compress air or sources of gaseous oxygenused in liquid phase oxidation or other equipment requiring mechanicalwork. Such extracted energy can be used elsewhere in the process or inother processes. Alternatively, it can be stored or delivered to anelectrical grid for transmission to other locations. Exhaust gasremaining after recovery of energy as work can be vented, preferablyafter being subjected to additional treatments, for example condensationto remove water if present m appreciable amounts in the condensing zoneexhaust gas, and caustic scrubbing to remove bromine or other compoundswhich may be undesirable for atmospheric release, if desired, energyrecovery can be conducted after scrubbing or otherwise treating the gasfor removal of corrosive components. Removal of corrosive componentsbefore recovery of energy can be beneficial in allowing internalcomponents of an expander or other power recovery device to beconstructed of less corrosion-resistant materials than might otherwisebe preferred; however, treatment for removal of such components also canreduce power recoverable from the gas.

As an alternative to recovering energy from a condensing zone highpressure exhaust gas or, more preferably, as an additional steppreceding recovery of energy as in the form of work as described above,exhaust gas from condensation can be treated for removal of organic andother combustible compounds and corrosive components. Such treatments,in some embodiments, are particularly useful for recovering minoramounts of reaction products of solvent monocarboxylic acid fromoxidation as well as traces of unreacted aromatic hydrocarbon feedmaterial that may remain in the exhaust gas. In embodiments of theinvention in which condensation of high pressure gas from separationincludes one or more condensations at a temperature low enough thatwater in the gas is substantially, and preferably at least about 80%,condensed and volatile impurities such as lower alcohol and esterreaction products of the solvent monocarboxylic acid are substantiallyretained in an uncondensed exhaust gas phase that is cooledsufficiently, preferably to a temperature in the range of about 40 toabout 90° C., treatment for recovery of such impurities is facilitatedbecause the uncondensed exhaust gas from, condensation is cool enoughfor use of liquid scrubbing agents for recovery. In other embodiments,treatment is beneficial to reduce or eliminate organic species such assuch unreacted feed material and solvent by-products if not removedotherwise, as well as corrosive alkyl bromide reaction by-products fromliquid phase oxidations in which a source of bromine is used as promoterfor the liquid phase oxidation catalyst and carried over into the highpressure vapor phase generated in the liquid phase oxidation and, inturn, into the high pressure gas removed from separation and exhaust gasremoved from condensation. It will be appreciated that such treatmentscan affect the amount of energy recoverable from the exhaust gas aftercondensation. Accordingly, in embodiments of the invention in whichcondensing zone exhaust gas is treated before recovery of energy in theform of work, preferred treatments are conducted without substantialloss of pressure or volume of the gas. When condensing zone exhaust gashas appreciable water content it. also is preferred that any suchtreatment be conducted without appreciable condensation of water fromthe gas or cooling to such an extent that recovery of energy in the formof work results in significant condensation of water. In suchembodiments, pre-heating of the treated gas before recovery of energymay be beneficial.

In embodiments of the invention comprising treating a pressurizedexhaust gas from condensation for removal of unreacted feed material andsolvent by-products generated in the liquid phase oxidation, such aslower alkyl esters of the solvent monocarboxylic acid, treatment isbeneficial in allowing for return of such components to oxidation.Treatment also can reduce presence of such impurities in process recyclestreams and steady state equilibrium levels thereof in overall processoperation. Uncondensed gas under pressure removed from condensation canbe contacted, preferably at a temperature of about 35 to about 60° C.,with liquid scrubbing agent to provide a scrubbed gaseous phase withreduced levels of aromatic feed material and/or solvent by-products anda liquid product comprising the scrubbing agent and enriched in at leastone of unreacted aromatic feed material and solvent monocarboxylic acidreaction products from liquid phase oxidation. The liquid product ispreferably returned to the reaction zone in a liquid phase oxidationstep. Scrubbing can be accomplished using any suitable scrubbing deviceand scrubbing agents for contacting a gas stream comprising the highpressure condensation exhaust gas to remove volatile components such asunreacted feed material and solvent monocarboxylic acid by-products fromoxidation from the gas into a liquid phase. High pressure absorptioncolumns with internal structure, such as trays or packed beds, forpromoting contact between gases to be scrubbed and liquid scrubbingagent are commonly utilized. Suitable scrubbing agents are materialsthat are liquid at the temperature of the gas to be scrubbed and inwhich the materials to be recovered have substantial solubility.Examples include lower alcohols and C.sub. 1-8 carboxylic acids such asacetic acid, propionic acid, butyric acid and tire like, A preferredliquid scrubbing agent is the monocarboxylic acid used as solvent forliquid phase oxidation and mixtures thereof with water, Suitablescrubbing agents, equipment and use thereof for recovery of off-gascomponents from liquid phase oxidation of aromatic feed materials toaromatic carboxylic acids are described in further detail in U.S. Pat.No. 6,143,925, which is incorporated herein by reference.

Pressurized condenser exhaust gas, with or without prior treatment asfor scrubbing of unreacted feed material or solvent by-products asdescribed above, can also be treated to remove corrosive or othercombustible materials. While any means for such removal withoutsubstantial loss of pressure and volume of the gas can be employed, thegas preferably is subjected to an oxidation process, and most preferablya catalytic oxidation process for removal of organic, combustible andcorrosive components. Such treatments generally comprise heating anuncondensed gas under pressure, and comprising exhaust gas underpressure removed from condensation or after scrubbing or othertreatment, and gaseous oxygen in a combustion zone under pressure notsubstantially less than that of the pressurized gas and at elevatedtemperature effective to oxidize organic, combustible and corrosivecomponents to a less corrosive or more environmentally compatible gascomprising carbon dioxide and water. Heating under pressure with oxygengas preferably is conducted in the presence of a suitable oxidationcatalyst disposed within the combustion zone so as not to interrupt flowof the pressurized gas therethrough. The pressurized gas can optionallybe subjected to preheating before oxidation. Preheating can beaccomplished by any suitable means such as by heat exchange, directsteam injection or other suitable means. Optionally, combustiontreatment can also include scrubbing a pressurized gas removed fromcombustion to remove acidic, inorganic materials such as bromine andhydrogen bromide which are generated by oxidation of alkyl bromidespresent in the condenser exhaust gas when a bromine source is used forliquid phase oxidation as noted above.

Catalysts for catalytic oxidation generally comprise at least onetransition group element of the Periodic Table (IUPAC). Group VIIImetals are preferred, with platinum, palladium and combinations thereofand with one or more additional or adjuvant metals being especiallypreferred. Such catalyst metals may be used in composite forms such asoxides. Typically, the catalyst metals are disposed on a support orcarrier material of lower or no catalytic activity but with, sufficientstrength and stability to withstand the high temperature and pressureoxidizing environment of the combustion zone. Suitable catalyst supportmaterials include metal oxides comprising one or more metals, examplesof which include mullite, spinels, sand, silica, alumina silica alumina,titania and zirconia. Various crystalline forms of such materials can beutilized, such as alpha, gamma, delta and eta aluminas, and rutile andanatase titanias. Catalyst metal loadings on support compositions aresuitably fractions to several percents by weight, with higher loadingsbeing preferred for use when treating gases with significant water vaporcontent, such as about 20 vol. % or more. Catalysts can be used in anyconvenient configuration, shape or size. For example, the catalyst canbe in the form of pellets, granules, rings, spheres, and the like andpreferably may be formed into or disposed on a rigid cellular,honeycomb, perforated or porous structural configuration to promotecontact with gases present in the combustion zone without impeding gasflow through the zone. Specific examples of catalytic oxidationcatalysts for combustion treatment of exhaust gas removed fromcondensation in off-gas treatment according to the invention compriseabout one-half to about one wt % palladium supported on an aluminamonolith support.

In embodiments of the invention in which energy in the form of work isrecovered from gas comprising exhaust gas removed from a condensingzone, and especially when such a gas comprises appreciable water, e.g.,at least about 5 vol. %, the gas can optionally be heated to guardagainst presence of liquid water in the gas directed to energy recovery.Such heating can take place before, after or in combination with othertreatments or treatment steps such as thermal or catalytic oxidations.In such embodiments, heating can be accomplished by any suitabletechnique, such as by heat exchange or direct injection of steam orother heated gas. Heating to about 200° C. or greater is effective foravoiding condensation of water, with temperatures of about 250 to about350° C. preferred.

In addition to the condensing zone exhaust gas remaining aftercondensation of high pressure gas removed from the separation zone,condensation according to an off-gas treatment step of the inventedprocess results in condensation of a liquid from the pressurized gas.The condensate liquid comprises water of substantial purity as describedabove and according to preferred embodiments of the invention, isdirected at least in part to the separation zone such that reflux liquidsupplied to the separation zone comprises such condensate liquid. Thecondensate liquid also is statable for other uses such as wash liquidfor solid-liquid separations of impure aromatic carboxylic acid productsfrom liquid phase oxidation. As between the condensate liquid and thewater-enriched second liquid phase removed from off-gas separationaccording to the invented process, the second liquid phase is preferredfor use in integrated process that include purification of impurearomatic carboxylic acids, such as are recovered from liquid phaseoxidation, due to lower solvent monocarboxylic acid oxidation by-productcontent than in the condensate liquid recovered from the second highpressure vapor phase from separation.

Purification

In embodiments of the invention comprising purification or manufactureof purified aromatic carboxylic acids, purification comprises at leastone step that comprises contacting with hydrogen at elevated temperatureand pressure in the presence of a catalyst comprising a hydrogenationcatalyst metal a purification reaction solution comprising a liquid thatcomprises water and has dissolved therein aromatic carboxylic acid andimpurities to form a purification liquid reaction mixture comprising thearomatic carboxylic acid and hydrogenated impurities dissolved in aliquid comprising water. In preferred embodiments, a purificationreaction solution is formed by dissolving in a liquid comprising water acrude solid product recovered from liquid phase oxidation comprisingaromatic carboxylic acid and impurities comprising oxidation by-productsof the aromatic feed material for the oxidation. Pure forms of aromaticcarboxylic acid product containing reduced levels of impurities can berecovered from the purification liquid reaction mixture, preferably bycrystallization, and the resulting pure form of product can be separatedfrom a liquid purification mother liquor remaining after recovery of thepure form of product and/or from one or more liquids comprising water,such as crystallization solvents and wash liquids. The inventionincludes embodiments in which at least one liquid comprising water thatis used in the purification comprises a water-enriched second liquidphase removed from the separation zone of an off-gas separationaccording to the invention. As indicated above, in other embodimentspurification mother liquor from at least one purification is directed tooff-gas separation for introduction to the separation zone as refluxliquid comprising water. As described above, aromatic carboxylic acidproducts obtained by liquid phase oxidation of feed materials comprisingaromatic compounds with oxidizable substituents, also sometimes referredto as a crude aromatic carboxylic acid product or crude product fromliquid phase oxidation, comprise aromatic carboxylic acid and one ormore oxidation intermediates or by-products. Although specific chemicalcompositions of intermediates and by-products vary depending compositionof the oxidation, feed material, oxidation reaction conditions and otherfactors, and even for given feed materials may not be fully known, theyare known to comprise one or more aromatic carbonyl compounds, such asbenzaldehydes, carboxybenzaldehydes, fluorenones and anthraquinones,that cause or correlate with undesirable color of desired aromaticcarboxylic acid products or of polyesters made therefrom and can behydrogenated to species more soluble in aqueous solution than thearomatic carbonyl compounds and aromatic carboxylic acid or to specieswith less color or color-forming tendencies. Preferred impure aromaticcarboxylic acid products to be purified according to embodiments of tireinvention are crude product comprising aromatic carboxylic acid andby-products produced by liquid phase oxidation of aromatic feed materialin a liquid phase oxidation, and most preferably continuous processes inwhich liquid phase oxidation and purification steps are integrated suchthat a crude solid product of liquid phase oxidation is a startingmaterial for the purification. However, it also will be appreciated thatthe starting material for purification can be or include an impureproduct comprising an aromatic carboxylic acid and aromatic carbonylimpurities as described above, whether present or generated asby-products from an integrated or non-integrated liquid phase oxidationof aromatic feed material or from other processes or sources. Thus, theinvention includes embodiments in which an impure aromatic carboxylicacid product starting material for purification comprises aromaticcarboxylic acid and at least one aromatic carbonyl impurity that tonus ahydrogenated, carbonyl-substituted aromatic product with greatersolubility in aqueous solution or less color or color-forming tendenciesthan the unhydrogenated aromatic carbonyl impurity. Impure forms ofaromatic carboxylic acid product suitable as starting materials forpurification, including crude products recovered from a liquid phaseoxidation according to embodiments of the invention, also may containminor amounts of solvent monocarboxylic acid residues that remain in theimpure product. Amounts ranging from several hundred to thousands ppmwas commonly present in products from commercial scale liquid phaseoxidations do not adversely affect purification according to theinvented process. Most preferably, solvent monocarboxylic acid contentof an aromatic carboxylic acid product to be purified does not exceedabout 10 wt %.

In greater detail, a preferred purification step according to theinvention comprises dissolving in a liquid comprising water, at least aportion of which most preferably comprises a second liquid phasecomprising water removed from off-gas separation according to theinvention, a solid product comprising aromatic carboxylic acid andimpurities to form a purification reaction solution, contacting thepurification solution at elevated temperature and pressure with hydrogenin the presence of a hydrogenation catalyst to form a purificationliquid reaction mixture, recovering from the purification liquidreaction mixture a solid purified product comprising aromatic carboxylicacid with reduced levels of impurities and separating an aqueous liquidpurification mother liquor comprising oxidation by-products,hydrogenation products thereof and combinations thereof from therecovered solid purified product.

Hydrogenation of impure aromatic carboxylic acids to reduce impuritieslevels is conducted with the impure acid in aqueous solution. Apreferred solvent for the purification solution in some embodiments ofthe invention comprises a second liquid phase removed from theseparation zone of an off-gas separation according to the invention.Supply of second liquid phase directly from separation and without addedor intermediate treatments for removal of by-products or impurities ispreferred in continuous and integrated process operations to avoidcosts, complexities and additional equipment for added handling, storageor treatment of the condensate liquid, although it will be appreciatedthat such added treatments, while unnecessary to render the secondliquid phase suitable as a solvent for purification, are not precluded.Similarly, while unnecessary for obtaining a liquid of sufficient purityfor use as purification solvent according the invention, it will beappreciated that the invention contemplates use of other suitable watersources such as fresh demineralized water or other purified sources ofwater in addition to or as alternatives to the second liquid phase fromoff-gas separation. Preferably the water-enriched second liquid phasefrom separation according to the invention makes up at least about 50%of the solvent for the purification reaction solution and morepreferably about 80 to about 100%.

Concentrations in the purification solvent of impure aromatic carboxylicacid to be treated in a purification step generally are low enough thatthe impure acid is substantially dissolved and high enough for practicalprocess operations and efficient use and handling of liquid used assolvent and remaining as purification mother liquor after recovery of apure form of aromatic carboxylic acid with reduced impurities frompurification reaction mixtures. Suitably, solutions comprising about 5to about 50 parts by weight impure aromatic carboxylic acid per hundredparts by weight solution at process temperatures provide adequatesolubility for practical operations. Preferred purification reactionsolutions contain about 10 to about 40 wt %, and more preferably about20 to about 35 wt %, impure aromatic carboxylic acid at the temperaturesused for purification by catalytic hydrogenation.

Catalysts suitable for use in purification hydrogenation reactionscomprise one or more metals having catalytic activity for hydrogenationof impurities in impure aromatic carboxylic acid products, such asoxidation intermediates and by-products and/or aromatic carbonylspecies. The catalyst metal preferably is supported or carried on asupport material that is insoluble in water and unreactive with aromaticcarboxylic acids under purification process conditions. Suitablecatalyst metals are the Group VIII metals of the Periodic Table ofElements (TUPAC version), including palladium, platinum, rhodium,osmium, ruthenium, iridium, and combinations thereof. Palladium orcombinations of such metals that include palladium are most preferred.Carbons and charcoals with surface areas of several hundreds orthousands m.²/g surface area and sufficient strength and attritionresistance for prolonged use under operating conditions are preferredsupports. Metal loadings are not critical but practically preferredloadings are about 0.1 wt % to about 5 wt % based on total weight of thesupport and catalyst metal or metals. Preferred catalysts for conversionof impurities present in impure aromatic carboxylic acid productscomprising crude terephthalic acid obtained by liquid phase oxidation ofa feed material comprising para-xylene contain about 0.1 to about 3 wt %and more preferably about 0.2 to about 1 wt % hydrogenation metal. Forsuch uses, the metal most preferably comprises palladium.

For practical applications, catalyst is most preferably used inparticulate form, for example as pellets, extrudate, spheres orgranules, although other solid forms also are suitable. Particle size ofthe catalyst is selected such that a bed of catalyst particles is easilymaintained in a suitable purification reactor but permits flow of thepurification reaction mixture through the bed without undesirablepressure drop. Preferred average particle sizes are such that catalystparticles pass through a 2-mesh screen but are retained on a 24-meshscreen (U.S. Sieve Series) and, more preferably, through a 4-mesh screenbut with retention on a 12-mesh and, most preferably 8-mesh, screen,

Contacting aqueous purification reaction solution with hydrogen in thepresence of catalyst tor purification is conducted at elevatedtemperatures and pressures. Temperatures range from about 200 to about370° C., with about 225 to about 325° C. being preferred and about 240to about 300° C. being most preferred. Pressure is at a level sufficientto maintain a liquid phase comprising the aqueous reaction solution.Total pressure is at least equal to, and preferably exceeds, the sum ofthe partial pressures of the hydrogen gas introduced to the process andwater vapor that boils off from the aqueous reaction solution at thetemperature of operation. Preferred pressures are about 35, and morepreferably about 70, to about 105 kg/cm.².

The aqueous purification reaction solution is contacted with hydrogengas under hydrogenation conditions as described above in a suitablereaction vessel capable of withstanding reaction temperatures andpressures and also the acidic nature of its liquid contents, A preferredreactor configuration is a cylindrical reactor with a substantiallycentral axis which, when the reactor is positioned for process use, isvertically disposed. Both upflow and downflow reactors can be used.Catalyst typically is present in the reactor in one or more fixed bedsof particles maintained with a mechanical support for holding thecatalyst particles in the bed while allowing relatively free passage ofreaction solution therethrough. A single catalyst bed is often preferredalthough multiple beds of the same or different catalyst or a single bedlayered with different catalysts, for example, with respect to particlesize, hydrogenation catalyst metals or metal loadings, or with catalystand other materials such as abrasives to protect the catalyst, also canbe used and may provide benefits. Mechanical supports in the form offlat mesh screens or a grid formed from appropriately spaced parallelwires are commonly employed. Other suitable catalyst retaining meansinclude, for example, a tubular Johnson screen or a perforated plate.Internal components and surfaces of the reactor and the mechanicalsupport for the catalyst bed are constructed of materials that aresuitably resistant to corrosion from contact with the acidic reactionsolution and reaction product mixture. Most suitably, supports forcatalyst beds have openings of about 1 mm or less and are constructed ofmetals such as stainless steel, titanium or Hastelloy C.

In preferred embodiments of the invention, aqueous solution of impurearomatic carboxylic acid to be purified is added to the reactor vesselat elevated temperature and pressure at a position at or near the topportion of the reactor vessel and the solution flows downwardly throughthe catalyst bed contained in the reactor vessel in the presence ofhydrogen gas, wherein impurities are reduced with hydrogen, in manyeases to hydrogenated products with greater solubility in the reactionmixture than the desired aromatic carboxylic acid or with less color orcolor-forming tendencies. In such a preferred mode, a liquidpurification reaction mixture comprising aromatic carboxylic acid andhydrogenated impurities is removed from the reactor vessel at a positionat or near a lower portion or bottom of the reactor.

Reactors used for purification may be operated in several modes. In onemode, a predetermined liquid level is maintained in the reactor and, fora given reactor pressure, hydrogen is fed at a rate sufficient tomaintain the predetermined liquid level. The difference between theactual reactor pressure and the vapor pressure of the vaporizedpurification solution present in the reactor head space is the hydrogenpartial pressure in the head space. Alternatively, hydrogen can be fedmixed with inert gas such as nitrogen or water vapor, in which case thedifference between the actual reactor pressure and the vapor pressure ofthe vaporized reaction solution present is the combined partial pressureof hydrogen, and the inert gas mixed therewith. In such cases hydrogenpartial pressure may be calculated from the known relative amounts ofhydrogen and inert gas present in the mixture.

In another operating mode, the reactor can be filled with the aqueousliquid reaction solution so that there is essentially no reactor vaporspace but a hydrogen bubble at the top or in the head of the reactorthat expands or contracts in size to provide volume in the reactor headso that hydrogen added to the reactor is dissolved into the incomingpurification reaction solution. In such an embodiment, the reactor isoperated as a hydraulically full system with dissolved hydrogen beingfed to the reactor by flow control. The concentration of hydrogen insolution may be modulated by adjusting the hydrogen flow rate to thereactor. If desired, a pseudo-hydrogen partial pressure value may becalculated from the solution hydrogen concentration which, in turn, maybe correlated with the hydrogen flow rate to the reactor.

When operating such that process control is effected by adjusting thehydrogen partial pressure, the hydrogen partial pressure in the reactoris preferably in the range of about one-half to about 15 kg/cm.² gaugeor higher, depending on pressure rating of the reactor, impuritieslevels of the impure aromatic carboxylic acid, activity and age of thecatalyst and other considerations known to persons skilled in the art.In operating modes involving directly adjusting hydrogen concentrationin the feed solution, the solution usually is less than saturated withrespect to hydrogen and the reactor itself is hydraulically full. Thus,an adjustment of the hydrogen flow rate to the reactor will result inthe desired control of hydrogen concentration in the solution.

Space velocity, expressed as weight of the impure aromatic acid in thepurification reaction solution per weight of catalyst per hour, duringhydrogenation is typically about 1 hour⁻¹ to about 25 hour⁻¹, andpreferably about 2 hours⁻¹ to about 15 hours⁻¹. Residence time of thepurification liquid stream in the catalyst bed varies depending on thespace velocity.

Pure forms of aromatic carboxylic acid product with reduced levels ofimpurities relative to the crude or other impure aromatic carboxylicacid product used for preparing the purification solution is recoveredfrom the liquid purification reaction mixture. The purification reactionmixture, comprising aqueous reaction solvent having dissolved thereinaromatic carboxylic acid and hydrogenated aromatic impurities havinggreater solubility in the aqueous reaction liquid than theirunhydrogenated precursors, is cooled to separate a pure form of solidaromatic carboxylic acid with reduced impurities from the reactionmixture, leaving a liquid purification mother liquor having hydrogenatedimpurities dissolved therein. Separation is commonly achieved by coolingto a crystallization temperature, which is sufficiently low forcrystallization of the aromatic carboxylic acid to occur, therebyproducing crystals within the liquid phase. The crystallizationtemperature is sufficiently high so that dissolved impurities and theirreduction products resulting from hydrogenation remain dissolved in theliquid phase. Crystallization temperatures generally range up to 160° C.and preferably up to about 150° C. In continuous operations, separationnormally comprises removing liquid purification reaction mixture fromthe purification reactor and crystallization of aromatic carboxylic acidin one or more crystallization vessels. When conducted in a series ofstages or separate crystallization vessels, temperatures in thedifferent stages or vessels can be the same or different and preferablydecrease from each stage or vessel to the next. Crystallizationtypically also results in flashing of liquid from the purificationliquid reaction mixture, which can be recovered by condensation andrecycled to one or more of purification, one or more upstreamcrystallization stages or, in preferred embodiments of the invention, toseparation of solvent monocarboxylic acid and water vapor in a highpressure vapor phase from liquid phase oxidation. Liquid comprisingwater, which preferably comprises the water-enriched liquid recovered asa second liquid phase in an off-gas separation according to the inventedprocess, is preferably added to the crystallized product recovered frompurification liquid reaction mixture recovered in stagewisecrystallizations either directly or, more preferably, indirectly in oneor more wash liquids for the crystallized product.

Thereafter, crystallized, purified aromatic carboxylic acid product isseparated from the purification mother liquor, including hydrogenatedimpurities dissolved therein. Separation of the crystallized product iscommonly conducted by centrifuging or by filtration. A preferredseparation comprises pressure filtration of an aqueous slurry of pureforms of aromatic carboxylic acid and washing of filter cake resultingfrom filtration with a liquid comprising water as described in U.S. Pat.No. 5,175,355, which is incorporated herein by reference. Thewater-enriched second liquid phase from an off-gas separation asdescribed herein is a preferred liquid comprising water for use as washliquid for the pure form of aromatic carboxylic-acid.

Purification mother liquor remaining after recovery of solid purifiedaromatic carboxylic acid from the purification reaction mixturecomprises water and hydrogenated derivatives of by-products orimpurities present in the impure aromatic carboxylic acid startingmaterial. The mother liquor commonly also includes minor amounts ofaromatic carboxylic acid that remain in solution. Such hydrogenatedderivatives include compounds suitable for conversion to aromaticcarboxylic acid by liquid phase oxidation and, accordingly, in preferredembodiments of the invention, at least a portion of such hydrogenatedderivatives are transferred directly or indirectly to a liquid phaseoxidation. Residual aromatic carboxylic acid present in the motherliquor also can be transferred directly or indirectly to liquid phaseoxidation after separation from, or more preferably, together with, suchhydrogenated derivatives. Transfer of such derivatives and aromaticcarboxylic acid to oxidation is conveniently accomplished by directingat least a portion of a purification mother liquor remaining afterseparation of a solid pure form of aromatic carboxylic acid to a liquidphase oxidation step. Water content of purification mother liquor canupset water balance in oxidation unless water from purification motherliquor directed to oxidation is accounted for in other streams that maybe returned to oxidation. Transfer of hydrogenated impurities in apurification mother liquor, alone or preferably in combination witharomatic, carboxylic acid present in the mother liquor, to liquid phaseoxidation is preferably accomplished without upsetting water balance inthe oxidation. More preferably, at least a portion, and most preferablysubstantially all, of a liquid mother liquor remaining after separationof the solid purified aromatic, carboxylic acid from the liquidpurification reaction mixture is transferred directly or indirectly to aseparation zone of off-gas separation according to the invention whereit is used as reflux liquid as previously described. Purificationreactor and catalyst bed configurations and operating details andcrystallization and product recovery techniques and equipment useful inthe process according to this invention are described in further detailin U.S. Pat. Nos. 3,584,039, 4,626,598, 4,629,715, 4,782,181, 4,892,972,5,175,355, 5,354,898, 5,362,908 and 5,616,792 which are incorporatedherein by reference.

FIG. 1 illustrates in further detail embodiments of a process formanufacture of aromatic carboxylic acids and apparatus for off-gasseparation according to the invention. While the figure illustrates, andis described with specific reference to, manufacture of a selectedaromatic carboxylic acid, terephthalie acid, by liquid phase oxidationof para-xylene as a preferred feedstock in a liquid phase reactionmixture comprising water and acetic acid as solvent monocarboxylic acidfor the oxidation and off-gas separation of acetic acid, water andby-products of the oxidation, and additional preferred embodiments andfeatures of the invention according to which oxidation and off-gasseparation are integrated with additional steps including recovery andseparation of a crude product from the liquid phase oxidation,purification of liquid phase oxidation product and various additionalby-product and energy recoveries, it will be understood thatspecific-embodiments, features, details and preferences are described toaid in understanding the invention but not to limit the invention or itsfeatures in any aspect, or embodiment.

The process illustrated in FIG. 1 also reflects preferred embodiments ofthe invented process in which liquid phase oxidation, off-gas separationand purification are integrated such that crude aromatic carboxylic acidproduct from the liquid phase oxidation is directed to purification foruse to form a purification solution, a high pressure off-gas from theoxidation is directed to the off-gas separation, liquid phase from theoff-gas separation is used as a purification liquid, and reflux liquidto the separation comprises mother liquor from the purification;however, it will be understood that the invention is not to beconsidered limited to the particular integration scheme represented inthe figure and that various multiple train, shared train and otherintegrated and non-integrated configurations are contemplated accordingto the invention. By way of illustrative examples, product comprisingaromatic carboxylic acid and reaction by-products from, multiple liquidphase oxidations can be directed to a single purification step in whicha liquid phase recovered in off-gas separation of a high pressure vaporphase from one or more of those or other liquid phase oxidations isdirected for use as a process liquid. As additional such examples, crudeproduct from a single liquid phase oxidation can be purified in separatepurification trains operated in. parallel, with high pressure vaporphase from the liquid oxidation subjected to off-gas separation forrecovery of a water-enriched liquid phase substantially free of solventby-products and transfer thereof to either or both such purificationtrains, or as an alternative or in addition, to a process in whichimpure aromatic carboxylic acid from a separate oxidation or process ispurified in a purification process or process steps as described herein.

The figure also represents a separation apparatus according to theinvention and also according to further embodiments of the invention inwhich the apparatus is integrated with other equipment such as areaction vessel for liquid phase oxidation.

Liquid and gaseous streams and materials used and present in the processrepresented in FIG. 1 are typically directed and transferred throughsuitable transfer lines, conduits and piping constructed of appropriatematerials for process use and safety. It will be understood thatparticular elements may be physically juxtaposed and can, whereappropriate, have flexible regions, rigid regions or both, in directingstreams or compounds, intervening apparatus or optional treatments canbe included. For example, appropriate pumps, valves, manifolds, gas andliquid flow meters and distributors, sampling and sensing devices andother equipment for monitoring, controlling, adjusting and divertingpressures, flows and other operating parameters may be present.

Referring to the figure, separation apparatus 330 is a columnarstructure that defines an enclosed interior space and is adapted forreceipt of a high pressure vapor phase removed from oxidation reactor110 in stream 111 and for removal of a second high pressure vapor phasethrough gas outlet 334. It also includes inlets as at 336 and 344 torintroduction of reflux liquids supplied from external sources such as instreams from other process steps or from holding vessels. An outlet asat 345 is positioned intermediately relative to reflux inlets 336 and344 for removal of a second liquid phase collected in the column.Structure in the interior space of the column and positionedintermediately between an inlet for receipt of the high pressure vaporphase from oxidation reactor 110 and reflux inlet 336 provides afractionating zone in the interior.

The separation device is designed so that in operation it is capable ofsubstantially separating C₁₋₈ monocarboxylic acid and water in the highpressure and temperature oxidation reactor overhead gas introduced tothe device and preferentially apportioning by-products of the liquidphase oxidation such that a first liquid phase rich in themonocarboxylic acid, a second liquid phase rich in water butsubstantially free of the solvent and by-products thereof generated inthe liquid phase oxidation and a second high pressure vapor phasecomprising water and substantially free of solvent, and by-products ofthe aromatic feed to liquid phase oxidation are formed. In preferredembodiments, direct association or close coupling of the oxidationreactor and separation device are effectuated by connection directly orby suitable pressure rated piping or other conduits between one or morevents in the oxidation reaction, vessel and one or more gas inlets to aseparation device, such that a vapor phase under liquid phase reactionconditions is removed from the reaction vessel and introduced into theseparation device at the same or substantially the same temperature andpressure as in the reaction zone.

A fractionating zone of the separation apparatus is configured with aplurality of theoretical equilibrium stages such as can be provided byinternal trays, structured packing, combinations of trays and packing,or other structure or combinations thereof providing surfaces within theinterior of the device for mass transfer between gaseous and liquidphases present in the device. At least about 20 theoretical equilibriumstages are provided. Separation efficiency increases with increasingtheoretical equilibrium stages, other things being equal, so there is notheoretical upper limit to the number of equilibrium stages that may beincluded in the separation apparatus used according to the invention.However, for practical purposes, separation such that solventmonocarboxylic acid in the high pressure vapor phase introduced to theseparation device is substantially removed into a liquid phase can heaccomplished with at least about 20, and preferably at least about 25theoretical equilibrium stages, while separation beyond that provided byabout 100 such stages make additional stages impractical or economicallyinefficient.

A preferred separation device with structured packing has at least about3 beds or zones of packing, and more preferably about 4 to about 8 suchbeds, to provide adequate surface and theoretical equilibrium stages forseparation. An example of a suitable packing material is Flexipacstructured packing, which is available from KGGP LLC in the form of thinsheets of corrugated metal arranged in a crisscrossing relationship tocreate flow channels and such that their intersections create mixingpoints for liquid and vapor phases. A preferred separation device withtrays includes 30 to about 90 trays, at least about 70% of which arepositioned between an inlet for the high pressure gas introduced to theseparation device from the reaction vessel, as best seen in FIG. 2 at338, and at least one reflux liquid inlet. Trays in the form or sieve orbubble cap trays are preferred and preferably have separationefficiencies of about 30 to about 60%. The number of trays for a givennumber of theoretical equilibrium stages can be calculated by dividingthe number of stages by efficiency of the trays.

In process use, gas and liquid phases introduced into the separationdevice and present therein are at elevated temperatures and includewater, solvent monocarboxylic acid and other corrosive components, forexample, bromine compounds and their disassociation products such ashydrogen bromide that are present in an oxidation reaction overhead gaswhen the catalyst used for the oxidation includes a source of bromine.Therefore, in preferred embodiments of the invention, internal structureand other features of the separation apparatus that contact gases andliquids during process operation are constructed of suitable metals toresist corrosion and other damage due to such contact. Titanium metal isa preferred material of construction for such surfaces, including trays,packings or other structure of the fractionating zone. Titanium surfacesof such structure may be subject to undesirable accumulation of soliddeposits comprising iron oxides from impurities present in liquidscirculated through the equipment. Processes for controllingaccumulations of iron oxide deposits or content of soluble ironimpurities in process liquids are described in commonly assigned U.S.Pat. No. 6,852,879 and U.S. 2002/374719 which are incorporated herein byreference.

In the embodiment of the invention represented in the drawing,separation device 330 is a high pressure distillation column having aplurality of trays, individual examples of which are best seen at 333and 337 in FIG. 2. Also as seen in FIG. 2, the column comprises at leastone lower outlet, as at 332, for removal of liquid from the column, forexample to oxidation. Gas inlet 338 is positioned at a lower portion ofthe column for receipt of oxidation reactor off-gas and vent 334 islocated at an upper portion for removal of the second high pressurevapor phase as an exit gas, For stagewise separations according to theinvention, the region between gas inlet 338 and reflux liquid inlet 344includes trays providing theoretical equilibrium stages for substantialseparation of solvent monocarboxylic acid and water in the high pressurevapor phase removed from liquid phase oxidation in a first stage orportion of column 330. Trays positioned between reflux inlet 344 andsecond liquid outlet 345 and providing theoretical equilibrium stagesfor separation of by-products of an aromatic feed material to oxidationand water to apportion such by-products to a refluxing liquid phaseprovide a second portion of a separation zone in the column. Trayspositioned between liquid outlet 345 and reflux inlet 336, such as areillustrated at 333 and 337, provide theoretical stages for separation ofsolvent monocarboxylic acid oxidation by-products and water in a thirdportion of separation zone. Liquid outlet 332 is positioned for removalas a bottoms liquid of a first liquid phase that is enriched in solventmoncarboxylic acid separated from the oxidation off-gas in a firstportion of the separation zone. A tray configured with a boot, trough,accumulation channel or other collection means at a circumferentialboundary thereof as at 339 is in flow communication with liquid outlet345 and adapted tor collection of a second liquid phase of refluxingliquid flowing through separation for removal through outlet 345. Outlet345 in combination with associated internal structure of the separationapparatus for collecting a refluxing liquid phase at or from betweentrays or packing beds or other structure of a fractionating zone, suchas collection means 339, provide a side draw from the column forcollecting and removing a water-enriched, second liquid phase recoveredin the apparatus.

Referring again to FIG. 1, the separation apparatus is adapted toreceive a high pressure vapor phase from liquid phase oxidation reactionzone 110. In some embodiments, apparatus according to the inventioncomprises a separation apparatus in combination with at least one liquidphase oxidation reactor in flow communication with the separationapparatus such that, a high pressure overhead gas removed from thevessel through at least one overhead gas vent, as at 116, is receivedinto the separation device. In such embodiments, reaction vessel 110preferably comprises a substantially cylindrical shell that defines asubstantially enclosed interior volume, in use, a lower portion of theinterior volume contains a liquid reaction body while an overheadreaction off-gas is contained in a portion of the interior volume abovethe liquid level. The interior volume is in communication with theexterior of the reaction vessel through a plurality of inlets, anexample of which is seen as 112 in FIG. 1, through which liquid aromaticfeed material, solvent and soluble forms of catalyst are introduced fromliquid charge vessels (not shown) and compressed air or another sourceof oxygen gas is introduced from a compressor or other suitable device(not shown) via suitable transfer lines (not shown). The inletspreferably are disposed such that liquid and gaseous components areintroduced below the liquid level in the interior of the vessel. Thereaction vessel also includes at least one outlet, as at 114, forremoving from the interior a liquid phase reaction mixture whichincludes a crude product comprising aromatic carboxylic acid andoxidation by-products. Reaction vessel 110 also comprises at least onevent or outlet as at 116 for removal from the vessel interior of a highpressure vapor phase evaporated from the liquid reaction body. Vent 116preferably is positioned to correspond to an upper portion of the vesselwhen it is in position for process use.

A preferred reaction vessel design is a substantially cylindrical vesselhaving a central axis extending substantially vertically when the vesselis positioned for process use. The vessel is adapted for use with astirring mechanism 120 comprising a shaft having one or more impeller'smounted thereon and capable of being rotated within the interior of thereaction vessel to stir the liquid reaction mixture present in thevessel during process use. In preferred embodiments of the invention, atleast two impellers or mixing features are mounted on the shaft formixing of gaseous and liquid components within the liquid reaction bodywithout adverse settling of solids in lower portions of the vessel.Axial flow impellers, generally configured as propellers, radial flowmixers, such as flat blade disc turbines and disperser discs, helicalribbon mixing elements, pitched blade turbines with blades pitches forupward or downward flow, anchor-type mixers providing predominantlytangential flow and other configurations are suited for mixing theliquid phase oxidation reaction system and preferably are used invarious combinations to account for greater solids content in lowerregions of the liquid reaction mixture, greater gas content in upperregions and other characteristics of the liquid phase reaction mixturethat can vary throughout the liquid body. Other designs are disclosed inU.S. Pat. No. 5,198,156, describing mixing elements with radiallyextending, rotating blades mounted on a flat rotor and having a hollowblade configuration with a discontinuous leading edge, continuoustrailing edge, absence of external concave surfaces and an open outerend and preferably used in conjunction with a vertical pipe orperforated gas sparger for gas distribution, and U.S. Pat. No.5,904,423, which describes a mixer in which stirring elements aremounted at a downward angle on a central, rotating shaft and arewedge-shaped in the direction of movement through the liquid, withradial inner ends of the trailing edges of the blades angled outwardlyin the direction of motion of the blades, and used with features forintroducing a gas from below the stirring elements into a central cavityformed by a conical disk at an end of the shaft.

At least those portions of the reaction vessel, agitator shaft, andmixing elements that contact the liquid reaction mixture and overheadgas in process use are constructed of substantially corrosion resistantmaterials. Examples include titanium metal, which is preferred, alloysand duplex stainless steels.

According to the preferred process embodiment represented in FIG. 1liquid para-xylene feed material comprising at least about 99 wt %para-xylene, aqueous acetic acid solution, preferably containing about70 to about 95 wt % acetic acid, soluble compounds of cobalt andmanganese, such as their respective acetates, as sources oxidationcatalyst metals and of bromine, such as hydrogen bromide as promoter forthe catalyst and air are continuously charged to oxidation reactionvessel 110, which is a pressure rated, continuous stirred tank reactor,through inlets, one of which is depicted for purposes of illustration asat 112. Solvent and para-xylene feed are charged at rates providing asolvent to feed weight ratio of about 2:1 to about 5:1. Cobalt andmanganese sources preferably are used in amounts providing about 100 toabout 800 ppmw each based on weight of para-xylene feed material.Bromine preferably is used in an amount such that the atom ratio ofbromine to catalyst metals is about 0.1:1 to about 1.5:1.

Stirring is provided by rotation of agitator 120, the shaft of which isdriven by an external power source (not shown) causing impellers mountedon the shaft and located within the liquid body in the reactor toprovide forces for mixing of liquids and dispersion of gases within theliquid body and avoiding settling of solids in its lower regions.Catalyst and promoter, each preferably as a solution in acetic acidsolvent, are introduced into the liquid body in die reaction vessel. Airis supplied from below and within the sweep path of a lower impeller ata rate effective to provide at least about 3 moles molecular oxygen permole of aromatic, feed material.

Para-xylene oxidizes in the stirred liquid reaction mixture in reactor110, predominantly to terephthalic acid, but also reacts to formby-products including partial and intermediate oxidation products, suchas 4-carboxybenzaldehyde, 1,4-hydroxymethyl benzoic acid and p-toluicacid, and others such as benzoic acid. Solid reaction productscomprising terephthalic acid and para-xylene oxidation by-productsprecipitate from the liquid reaction mixture, with lesser amountsthereof remaining dissolved in the liquid. Solids content of the liquidslurry typically ranges up to about 50 wt. % and preferably from about20 to about 40 wt. %. Water is also generated as a product of theoxidation. The oxidation reaction is exothermic and heat generated bythe reaction causes boiling of the liquid phase reaction mixture andformation of an overhead vapor phase comprising vaporized acetic acid,water vapor and gaseous by-products from the oxidation reaction, carbonoxides, nitrogen from the air charged to the reaction and unreactedoxygen. The vapor phase may also include minor amounts of unreactedpara-xylene feed. The interior volume of reactor 110 is maintained underpressure sufficient to maintain the liquid phase nature of the reactionmixture, preferably at about 5 to about 21 kg/cm.² gauge. Overhead vaporis removed from the reactor through vent 116. The reactor contents aremaintained at an operating temperature in the range of about 160 toabout 225° C., based on the rate of removal of the vapor phase alsotaking into account temperatures and flow rates of streams removed fromand returned to the reactor as described below.

A liquid effluent comprising solid para-xylene oxidation products,including terephthalic acid, slurried in the liquid phase reactionmixture, which also contains dissolved para-xylene, oxidationby-products and catalyst metals, is removed from reaction vessel 110through slurry outlet 114 and directed in stream 115 to acrystallization zone for recovery of a solid product of the oxidationcomprising terephthalic acid and oxidation by-products of thepara-xylene feedstock.

In the embodiment of the invention illustrated in FIG. 1,crystallization is conducted in multiple stirred crystallizationvessels, 152 and 156 in series and in flow communication for transfer ofproduct slurry from vessel 152 to vessel 156. Cooling in thecrystallization vessels is accomplished by pressure release, with theslurry cooled in vessel 152 to a temperature in the range of about150-190° C. and then further to about 110-150° C. in vessel 156. One ormore of the crystallization vessels is vented, as at 154 and 158,respectively, for removal to heat exchange means (not shown) of vaporresulting from pressure let down and generation of steam from the Hashedvapor. Vapor removed from one or more upstream crystallization vessels,such as vessel 152, to heat exchange means is preferably condensed andliquid condensate comprising water, acetic acid solvent and solubleproducts and by-products of the oxidation can directed to one or moredownstream crystallization vessels, as at 156, to allow for recovery ofcrystallizable components such as terephthalie acid and oxidationby-products entering and condensed from the flashed vapors from one ormore upstream vessel.

Crystallization vessel 156 is in fluid communication with a solid-liquidseparation device 190, which is adapted to receive from thecrystallization vessel a slurry of solid product comprising terephthalieacid and oxidation by-products in a mother liquor from the oxidationcomprising acetic acid, and water, and to separate a crude solid productcomprising terephthalie acid and by-products from the liquid. Separationdevice 190 is a centrifuge, rotary vacuum filter or pressure filter. Inpreferred embodiments of the invention, the separation device is apressure filter adapted for solvent exchange by positive displacementunder pressure of mother liquor in a filter cake with wash liquidcomprising water. The oxidation mother liquor that results from theseparation exits separation device 190 in stream 191 for transfer tomother liquor drum 192. A major portion of the mother liquor istransferred from drum 192 to oxidation reactor 110 for return to theliquid phase oxidation reaction of acetic acid, water, catalyst andoxidation reaction by-products dissolved or present as fine solidparticles in the mother liquor. Crude solid product comprisingterephthalie acid and impurities comprising oxidation by-products of thepara-xylene feedstock is conveyed, with or without intermediate dryingand storage, from separation device 190 to purification solution make upvessel 202 in stream 197. The crude solid product is slurried in make upvessel 202 in purification reaction solvent, all or at least a portion,and preferably about 60 to about 100 wt. %, of which, comprises a secondliquid phase from an off-gas separation of water and acetic acid in avapor phase removed from reactor 110 to column 330 and by-products ofthe oxidation. If used, make up solvent, such as fresh demineralizedwater or suitable recycle streams such as liquid condensed from vaporsresulting from pressure letdown in crystallization of purifiedterephthalie acid product as discussed below, can be directed to make uptank 202 from vessel 204. Slurry temperature in the make up tankpreferably is about 80 to about 100° C.

Crude product is dissolved to form a purification reaction solution byheating, for example to about 260 to about 290° C. in makeup tank 202 orby passage through heat exchangers (not shown) as it is transferred topurification reactor 210. In reactor 210, the purification reactionsolution is contacted with hydrogen under pressure preferably rangingfrom about 85 to about 95 kg/cm.².

A portion of the purification liquid reaction mixture is continuouslyremoved from hydrogenation reactor 210 in stream 211 to crystallizationvessel 220 where terephthalic acid and reduced levels of impurities arecrystallized from the reaction mixture by reducing pressure on theliquid. The resulting slurry of purified terephthalic acid and liquidformed in vessel 220 is directed to solid-liquid separation apparatus230 in stream line 221. Vapors resulting from pressure letdown in thecrystallization reactor can be condensed by passage to heat exchangers(not shown) for cooling and the resulting condensate liquid redirectedto the process, for example as recycle to purification feed makeup tank202, through suitable transfer lines (not shown). Purified terephthalicacid exits solid-liquid separation device 230 in stream 231. Thesolid-liquid separation device can be a centrifuge, rotary vacuumfilter, a pressure filter or combinations of one or more thereof. Asecond liquid phase removed from column 330 can be directed to theseparation device as wash liquid for separation to replace or reducedemineralized water requirements for final washing of the purifiedproduct.

Purification mother liquor from which the solid purified terephthalicacid product is separated in solid-liquid separator 230 comprises water,minor amounts of dissolved and suspended terephthalic acid andImpurities including hydrogenated oxidation by-products dissolved orsuspended in the mother liquor. According to the preferred processembodiment illustrated in FIG. 1, at least a portion, and preferably allor substantially all, of the purification mother liquor is directed instream 233 to oxidation reaction off-gas separation in high pressuredistillation column 330 and introduced thereto. The purification motherliquor directed to column 330 is introduced to the column at a lowerportion thereof, as at 344. to provide liquid reflux for separation.Transfer of purification mother liquor from solid-liquid separationdevice 230 to the high pressure distillation column also allows forrecycle of terephthalic acid and impurities in the mother liquor, suchas benzoic acid and p-toluic acid by-products to oxidation reactor 110where they are oxidized or converted to terephthalic acid, while watercontent of the purification mother liquor vaporizes and refluxes in thedistillation column, exiting in a pressurized gas and/or a second liquidphase removed from column, without significantly impacting water balancein oxidation. Transfer of purification mother liquor from solid-liquidseparation device 230 to the distillation column also reduces the volumeof liquid effluent that needs to be directed to liquid waste treatmentand provides for return of valuable terephthalic acid to oxidation and,in turn, removal thereof tor recovery in oxidation crystallizers 152 and156.

Reaction off-gas generated by the liquid phase oxidation of para-xylenefeedstock in reactor vessel 110 is removed from the reactor through vent116 and directed in stream 111 to separation in column 330 which, asdepicted in FIG. 2, represents a high pressure distillation columnhaving a plurality of trays preferably providing about 28 to about 63theoretical plates and to which is supplied liquid for reflux throughliquid inlets 336 and 344. The vapor stream from oxidation is introducedto column 330 preferably at temperature and under pressure of about 150to about 225° C. and about 4 to about 21 kg/cm² gauge, respectively, andnot substantially less than in oxidation reactor 110, As describedabove, FIG. 1 illustrates a preferred embodiment in which reflux liquidintroduced to the column comprises purification mother liquor from whichsolid purified terephthalic acid is separated in solid-liquid separationdevice 230. Column 330 includes 80 trays, about 50 to about 70 of whichare disposed below reflux inlet 344, with the remainder positioned abovereflux inlet 344 but below introduction of a second reflux liquid at336. Inlets 336 and 344 are positioned so that they are separated bytrays corresponding to at least about three theoretical equilibriumstages, and preferably about 3 to about 20 such stages. According topreferred embodiments of the invention as represented in FIG. 1, refluxliquid supplied to the column at 336 is preferably a condensate liquidrecovered, by condensation of a high pressure and temperature secondvapor phase removed from distillation column 330 in condensing zone 350and directed to the column in stream 355, while reflux liquid suppliedat reflux inlet port 344 from stream 233 is preferably a purificationmother liquor directed to the column for such use from solid-liquidseparation of a purified product from the liquid phase oxidation. Refluxsupplied to the column at inlet preferably provides about 70 to about85% of the volumetric flow of reflux liquid added to the column atinlets 344 and 336.

A first liquid phase rich in acetic acid solvent, for the liquid phaseoxidation recovered from the high pressure inlet gas to column 330together with para-xylene oxidation by-products such as benzoic acid andp-toluic apportioned to the liquid phase in column 330 is collected at alower portion of the column. A second liquid phase, which ispredominantly water but also contains minor amounts of benzoic acid andp-toluic acid by-products apportioned to the liquid phase as well asoxygen and methyl bromide, is collected and removed from the column atside draw outlet 345. A second high pressure vapor comprising watervapor, incondensable components of the oxidation off-gas and acetic acidby-products such as methanol and methyl acetate preferentiallyapportioned to the gas phase is removed from the column as an exit gasthrough overhead vent 334.

The acetic acid-rich first liquid phase resulting from separation indistillation column 330 exits the column at a lower portion thereof andpreferably is returned directly or indirectly to oxidation reactor 110,as in stream 331. Return of the liquid phase to oxidation provides makeup solvent acetic acid to the oxidation reaction and reduces feedstockloss by allowing for conversion to desired products of intermediates andby-products condensed from the oxidation vapor phase as well as thoserecycled from purification mother liquor reflux to the column. Thesecond liquid phase withdrawn from the column at side draw outlet 345 isdischarged into flash vessel 346 which Is at ambient or atmosphericpressure to remove corrosive components such as dissolved oxygen andmethyl bromide prior to being directed to purification solution makeupvessel 202 in stream 357 for use in forming the crude product slurry andpurification reaction solution that, is directed to purification reactor210. Other purification vessels and liquid receiving equipment and usesto which the water-enriched second liquid phase can he directed to afterflashing include crystallization vessel 220 for use as clean make-upsolvent to replace purification reaction liquid vaporized in thecrystallizer and solid liquid separation device 230 for use as washliquid or seal flush. The condensate liquid also is suitable for usesoutside a purification step, such as wash liquid for solvent exchangefilters.

Exit gas withdrawn from the column at vent 334 is directed to condensingmeans 350, which as depicted in FIG. 1, includes condensers 352 and 362,and disengagement drum 372. Preferably, condensation is conducted suchthat liquid condensate water at a temperature of about 40 to about 60°C. is recovered in at least one stage. In the embodiment illustrated inthe figure, condensation is conducted by indirect heat exchange incondensing means 352. with water at a temperature of about 120 to about170° C. and the resulting liquid condensate is directed to column 330 instream 355 for addition at reflux inlet 336. Liquid and uncondensed gasfrom condenser 352 is directed to condenser 362 in stream 361 forcondensation using cooling water at about 30 to about 40° C. Gas andliquid effluent from condenser 362 is directed in stream 363 to drum 372in which condensate liquid comprising water is collected and removed instream 373, which can be directed to other uses such as seal flushliquid or to a purge stream. A condenser exhaust gas under pressure iswithdrawn as in stream 375.

Water used as heat exchange fluid for condensation of the second highpressure gas from distillation column 330 is heated by heal exchange incondensing means 350 to generate pressurized steam which can be directedto an energy recovery device such as steam turbine 450 in the processembodiment depicted in FIG. 1. Condensation using two or more condensersin series using heat exchange fluids at successively lower temperaturesallows for generation of steam at different pressures, thereby allowingfor efficiencies in use of steam at the different pressures by matchingwith differing heat or energy inputs to operations in which steam isused.

Uncondensed exhaust gas from condensation removed in stream 375comprises incondensable components such as unconsumed oxygen fromoxidation, nitrogen from the air used as oxygen source to the oxidation,carbon oxides from such air as well as from reactions in oxidation, andtraces of unreached para-xylene and its oxidation by-products, methylacetate and methanol, and methyl bromide formed from the brominepromoter used in oxidation. In the embodiment illustrated in the figure,the uncondensed gas is substantially free of water vapor owing tosubstantially complete condensation into the condensate liquid recoveredin the condensing means.

Uncondensed exhaust gas from condensing means 350 is under pressure ofabout 10 to about 15 kg/cm² and can be transferred directly to a powerrecovery device or to a pollution control device for removing corrosiveand combustible species in advance of power recovery. As depicted inFIG. 1, uncondensed gas is first directed to treatment to removeunreacted feed materials and traces of solvent acetic acid and/orreaction products thereof remaining in the gas. Thus, uncondensed gas istransferred in stream 375 to high pressure absorber 380 for absorbingpara-xylene, acetic acid, methanol and methyl acetate withoutsubstantial loss of pressure. Absorption tower 380 is adapted forreceipt of the substantially water-depleted gas remaining aftercondensation and for separation of para-xylene, solvent acetic acid andits reaction products from oxidation from the gas by contact with one ormore liquid scrubbing agents. A preferred absorber configuration,illustrated in the figure, comprises tower 380 having a plurality ofinternally disposed trays or beds or structured packing (not shown) toprovide surface for mass transfer between gas and liquid phases. Inlets(not shown) for addition of scrubbing agent to the absorber in streams381 and 383, respectively, are disposed at one or more upper, and one ormore lower portions of the tower. The absorber also includes an uppervent 382 from which a scrubbed gas under pressure comprisingincondensable components of the inlet gas to the absorber is removed instream 385 and a lower outlet 384 for removal of a liquid acetic acidstream into which components from the gas phase comprising one or moreof para-xylene, acetic acid, methanol from a lower portion of the towerand directed to reaction vessel 110 for reuse of recovered components.

Pressurized gas removed from condensing means 350 or, as depicted inFIG. 1, from the vent 382 from the high pressure absorber, can bedirected to pollution control means, as at 390, for converting organiccomponents and carbon monoxide in the gas from the condenser or theabsorber to carbon, dioxides and water. A preferred pollution controlmeans is a catalytic oxidation unit adapted for receiving the gas,optionally heating it to promote combustion and directing the gas intocontact with a high temperature-stable catalyst disposed on a cellularor other support such that gas flow through the device is substantiallyunaffected. Overhead gas from absorber 380 is directed to pollutioncontrol system 390 which includes preheater 392 and catalytic oxidationunit 394. The gas is heated to about 250 to 450° C. in the preheater andpassed under pressure of about 10 to 15 kg/cm.sup.2 to oxidation unit394 where organic components and by-products are oxidized to compoundsmore suited for beneficial environmental management.

An oxidized high pressure gas is directed from catalytic oxidation unit394 to expander 400 which is connected to generator 420. Energy from theoxidized high pressure gas is converted to work in the expander 400 andsuch work is converted to electrical energy by generator 420. Expandedgas exits the expander and can be released to the atmosphere, preferablyafter caustic scrubbing and/or other treatments for appropriatelymanaging such releases.

1. A continuous process for preparing aromatic carboxylic acids by theexothermic liquid-phase oxidation reaction of an aromatic feedstockcompound wherein water is efficiently recovered from the exothermicliquid-phase oxidation reaction, which process comprises: (a) oxidizingan aromatic feedstock compound to an aromatic carboxylic acid in aliquid-phase reaction mixture comprising water, a low-molecular weightmonocarboxylic acid solvent, a heavy metal oxidation catalyst and asource of molecular oxygen, under reaction conditions which produce agaseous high pressure overhead stream comprising water, gaseousby-products, and gaseous low-molecular weight monocarboxylic acidsolvent; (b) separating in a high efficiency separation apparatus asolvent monocarboxylic acid-rich first liquid phase and a water-richsecond liquid phase comprising dissolved oxygen and methyl bromide; and(c) reducing the amount of at least one of dissolved oxygen and methylbromide present in the second liquid phase providing a treated secondliquid phase,
 2. Tire process of claim 1 wherein step (c) comprisesflashing the second liquid phase to reduce the amount of at least one ofdissolved oxygen and methyl, bromide.
 3. The process of claim 2, whereinflashing the second liquid phase results in a drop in pressure fromabout 5 kg/cm² to about 40 kg/cm² to about atmospheric or ambientpressure.
 4. The process of claim 2, wherein flashing tire second liquidphase results in a drop in pressure from about 10 kg/cm² to about 20kg/cm² to about atmospheric or ambient pressure.
 5. The process of claim2, wherein the amount of dissolved oxygen in the second liquid phase isreduced from an amount of about 2.2 ppmw to an amount of less than about1.0 ppmw after flashing.
 6. The process of claim 2, wherein the amountof dissolved oxygen in the second liquid phase is reduced from an amountof about 2.2 ppmw to an amount of less than about 0.5 ppmw afterflashing,
 7. The process of claim 2, wherein the amount of dissolvedoxygen m the second liquid phase is reduced from an amount of about 2.2ppmw to an amount of less than about 0.05 ppmw after flashing.
 8. Theprocess of claim 2, wherein the amount of dissolved oxygen in the secondliquid phase is reduced from an amount of about 2.2 ppmw to an amount ofless than about 0.006 ppmw after flashing.
 9. The process of claim 2,wherein the amount of dissolved methyl bromide in the second liquidphase is reduced from an amount of about 0.03 ppmw to an amount of lessthan about 0.02 ppmw after flashing.
 10. The process of claim 2, whereinthe amount of dissolved methyl bromide in the second liquid phase isreduced from an amount of about 0.03 ppmw to an amount of less thanabout 0.01 ppmw after flashing.
 11. The process of claim 2, wherein theamount of dissolved methyl bromide in the second liquid phase is reducedfrom an amount of about 0.03 ppmw to an amount of less than about 0.009ppmw after flashing.
 12. The process of claim 2, wherein the amount ofdissolved methyl bromide in the second liquid phase is reduced from anamount of about 0.03 ppmw to an amount of less than about 0.006 ppmwafter flashing.
 13. The process of claim 2 further comprises a step ofsteam stripping the second liquid phase after flashing.
 14. he processof claim 2 further comprises a step of stripping the second liquid phasewith nitrogen after flashing.
 15. The process of claim 1, wherein thetreated second liquid phase is suitable for use as liquid comprisingwater in one or more steps of a process for purifying impure forms ofaromatic carboxylic acid.
 16. The process of claim 1, wherein thetreated second liquid phase is suitable for use as a seal flush.